| **Employee Benefit Plans (Details) (USD $)In Thousands, unless otherwise specified**                                             | 12 Months Ended                                                                                                                                                                                                                                                                                     | 0 Months Ended                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Change in projected benefit obligation:**                                                                                      |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| Benefit obligation, January 1                                                                                                    | $ 529,788us-gaap_DefinedBenefitPlanBenefitObligation                                                                                                                                                                                                                                                | $ 583,294us-gaap_DefinedBenefitPlanBenefitObligation                                                                                                                                                        |   |   |
| Service cost                                                                                                                     | 6,523us-gaap_DefinedBenefitPlanServiceCost                                                                                                                                                                                                                                                          | 7,476us-gaap_DefinedBenefitPlanServiceCost                                                                                                                                                                  |   |   |
| Interest cost                                                                                                                    | 26,170us-gaap_DefinedBenefitPlanInterestCost                                                                                                                                                                                                                                                        | 23,957us-gaap_DefinedBenefitPlanInterestCost                                                                                                                                                                |   |   |
| Actuarial (gain) loss                                                                                                            | 114,694us-gaap_DefinedBenefitPlanActuarialGainLoss                                                                                                                                                                                                                                                  | (67,063)us-gaap_DefinedBenefitPlanActuarialGainLoss                                                                                                                                                         |   |   |
| Regular benefits paid                                                                                                            | (32,134)us-gaap_DefinedBenefitPlanBenefitsPaid                                                                                                                                                                                                                                                      | (17,876)us-gaap_DefinedBenefitPlanBenefitsPaid                                                                                                                                                              |   |   |
| Amendment                                                                                                                        | 3,342us-gaap_DefinedBenefitPlanPlanAmendments                                                                                                                                                                                                                                                       | 0us-gaap_DefinedBenefitPlanPlanAmendments                                                                                                                                                                   |   |   |
| Acquisition                                                                                                                      | 3,311us-gaap_DefinedBenefitPlanBusinessCombinationsAndAcquisitionsBenefitObligation                                                                                                                                                                                                                 | 0us-gaap_DefinedBenefitPlanBusinessCombinationsAndAcquisitionsBenefitObligation                                                                                                                             |   |   |
| Benefit obligation, December 31                                                                                                  | 651,694us-gaap_DefinedBenefitPlanBenefitObligation                                                                                                                                                                                                                                                  | 529,788us-gaap_DefinedBenefitPlanBenefitObligation                                                                                                                                                          |   |   |
| Accumulated benefit obligation                                                                                                   | 625,574us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation                                                                                                                                                                                                                                       | 505,942us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation                                                                                                                                               |   |   |
| **Change in plan assets:**                                                                                                       |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| Value of plan assets, January 1                                                                                                  | 404,335us-gaap_DefinedBenefitPlanFairValueOfPlanAssets                                                                                                                                                                                                                                              | 336,534us-gaap_DefinedBenefitPlanFairValueOfPlanAssets                                                                                                                                                      |   |   |
| Actual return on plan assets                                                                                                     | 43,662us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets                                                                                                                                                                                                                                            | 62,701us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets                                                                                                                                                    |   |   |
| Employer contributions                                                                                                           | 28,099us-gaap_DefinedBenefitPlanContributionsByEmployer                                                                                                                                                                                                                                             | 22,976us-gaap_DefinedBenefitPlanContributionsByEmployer                                                                                                                                                     |   |   |
| Benefits paid                                                                                                                    | (32,134)us-gaap_DefinedBenefitPlanBenefitsPaid                                                                                                                                                                                                                                                      | (17,876)us-gaap_DefinedBenefitPlanBenefitsPaid                                                                                                                                                              |   |   |
| Value of plan assets, December 31                                                                                                | 443,962us-gaap_DefinedBenefitPlanFairValueOfPlanAssets                                                                                                                                                                                                                                              | 404,335us-gaap_DefinedBenefitPlanFairValueOfPlanAssets                                                                                                                                                      |   |   |
| Defined Benefit Pension Plan                                                                                                     |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| **Change in projected benefit obligation:**                                                                                      |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| Benefit obligation, January 1                                                                                                    | 456,402us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                                                                                                         | 500,603us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                 |   |   |
| Service cost                                                                                                                     | 4,952us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                                                                                                                 | 5,527us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                         | 4,350us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |   |
| Interest cost                                                                                                                    | 23,068us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                                                                                                               | 20,897us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                       | 20,945us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |   |
| Actuarial (gain) loss                                                                                                            | 113,410us-gaap_DefinedBenefitPlanActuarialGainLoss/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                                                                                                         | (57,051)us-gaap_DefinedBenefitPlanActuarialGainLoss/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                |   |   |
| Regular benefits paid                                                                                                            | (14,752)us-gaap_DefinedBenefitPlanBenefitsPaid/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                                                                                                             | (13,574)us-gaap_DefinedBenefitPlanBenefitsPaid/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                     |   |   |
| Benefit obligation, December 31                                                                                                  | 583,080us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                                                                                                         | 456,402us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                 | 500,603us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |   |
| Accumulated benefit obligation                                                                                                   | 562,760us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                                                                                              | 440,666us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                      |   |   |
| **Change in plan assets:**                                                                                                       |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| Value of plan assets, January 1                                                                                                  | 404,335us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                                                                                                     | 336,534us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                             |   |   |
| Actual return on plan assets                                                                                                     | 43,662us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                                                                                                   | 62,701us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                           |   |   |
| Employer contributions                                                                                                           | 10,717us-gaap_DefinedBenefitPlanContributionsByEmployer/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                                                                                                    | 18,674us-gaap_DefinedBenefitPlanContributionsByEmployer/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                            |   |   |
| Benefits paid                                                                                                                    | (14,752)us-gaap_DefinedBenefitPlanBenefitsPaid/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                                                                                                             | (13,574)us-gaap_DefinedBenefitPlanBenefitsPaid/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                     |   |   |
| Value of plan assets, December 31                                                                                                | 443,962us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                                                                                                     | 404,335us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                             | 336,534us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |   |
| Post-Retirement Medical Plan                                                                                                     |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| **Change in projected benefit obligation:**                                                                                      |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| Benefit obligation, January 1                                                                                                    | 11,081us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                                                                                                       | 13,462us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                               |   |   |
| Service cost                                                                                                                     | 985us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                                                                                                                | 947us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                        | 817us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |   |
| Interest cost                                                                                                                    | 574us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                                                                                                               | 512us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                       | 569us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |   |
| Actuarial (gain) loss                                                                                                            | (2,092)us-gaap_DefinedBenefitPlanActuarialGainLoss/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                                                                                                      | (3,221)us-gaap_DefinedBenefitPlanActuarialGainLoss/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                              |   |   |
| Regular benefits paid                                                                                                            | (508)us-gaap_DefinedBenefitPlanBenefitsPaid/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                                                                                                             | (619)us-gaap_DefinedBenefitPlanBenefitsPaid/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                     |   |   |
| Amendment                                                                                                                        | (569)us-gaap_DefinedBenefitPlanPlanAmendments/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                                                                                                           | 0us-gaap_DefinedBenefitPlanPlanAmendments/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                       |   |   |
| Acquisition                                                                                                                      | 3,311us-gaap_DefinedBenefitPlanBusinessCombinationsAndAcquisitionsBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                                                                     | 0us-gaap_DefinedBenefitPlanBusinessCombinationsAndAcquisitionsBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |   |   |
| Benefit obligation, December 31                                                                                                  | 12,782us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                                                                                                       | 11,081us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                               | 13,462us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |   |
| Accumulated benefit obligation                                                                                                   | 12,782us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                                                                                            | 11,081us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                    |   |   |
| **Change in plan assets:**                                                                                                       |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| Employer contributions                                                                                                           | 508us-gaap_DefinedBenefitPlanContributionsByEmployer/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                                                                                                    | 619us-gaap_DefinedBenefitPlanContributionsByEmployer/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                            |   |   |
| Benefits paid                                                                                                                    | (508)us-gaap_DefinedBenefitPlanBenefitsPaid/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                                                                                                             | (619)us-gaap_DefinedBenefitPlanBenefitsPaid/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                                     |   |   |
| Post-Retirement Medical Plan | J.P. Morgan Retirement Plan Services (RPS)                                                        |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| **Change in projected benefit obligation:**                                                                                      |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| Defined Benefit Plan, Number of Employees Covered                                                                                |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   | 1,000gac_DefinedBenefitPlanNumberofEmployeesCovered/ us-gaap_BusinessAcquisitionAxis= gac_J.P.MorganRetirementPlanServicesRPSMember/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
| Post-Retirement Medical Plan | Other Noncurrent Liabilities With Offset To Goodwill | J.P. Morgan Retirement Plan Services (RPS) |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| **Change in projected benefit obligation:**                                                                                      |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| Defined Benefit Plan, Benefit Obligation, Period Increase (Decrease)                                                             |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   | 3,311us-gaap_DefinedBenefitPlanBenefitObligationPeriodIncreaseDecrease/ us-gaap_BalanceSheetLocationAxis= gac_OtherNoncurrentLiabilitiesWithOffsetToGoodwillMember/ us-gaap_BusinessAcquisitionAxis= gac_J.P.MorganRetirementPlanServicesRPSMember/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
| Supplemental Executive Retirement Plan                                                                                           |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| **Change in projected benefit obligation:**                                                                                      |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| Benefit obligation, January 1                                                                                                    | 62,305us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                                                                                                    | 69,229us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                            |   |   |
| Service cost                                                                                                                     | 586us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                                                                                                             | 1,002us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                   | 991us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember |   |
| Interest cost                                                                                                                    | 2,528us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                                                                                                          | 2,548us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                  | 2,912us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember |   |
| Actuarial (gain) loss                                                                                                            | 3,376us-gaap_DefinedBenefitPlanActuarialGainLoss/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                                                                                                     | (6,791)us-gaap_DefinedBenefitPlanActuarialGainLoss/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                           |   |   |
| Regular benefits paid                                                                                                            | (16,874)us-gaap_DefinedBenefitPlanBenefitsPaid/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                                                                                                       | (3,683)us-gaap_DefinedBenefitPlanBenefitsPaid/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                |   |   |
| Amendment                                                                                                                        | 3,911us-gaap_DefinedBenefitPlanPlanAmendments/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                                                                                                        | 0us-gaap_DefinedBenefitPlanPlanAmendments/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                    |   |   |
| Benefit obligation, December 31                                                                                                  | 55,832us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                                                                                                    | 62,305us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                            | 69,229us-gaap_DefinedBenefitPlanBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember |   |
| Accumulated benefit obligation                                                                                                   | 50,032us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                                                                                         | 54,195us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                 |   |   |
| **Change in plan assets:**                                                                                                       |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| Employer contributions                                                                                                           | 16,874us-gaap_DefinedBenefitPlanContributionsByEmployer/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                                                                                              | 3,683us-gaap_DefinedBenefitPlanContributionsByEmployer/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                       |   |   |
| Benefits paid                                                                                                                    | (16,874)us-gaap_DefinedBenefitPlanBenefitsPaid/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                                                                                                       | (3,683)us-gaap_DefinedBenefitPlanBenefitsPaid/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                                |   |   |
| Supplemental Executive Retirement Plan | Other Noncurrent Liabilities With Offset To Goodwill                                    |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| **Change in projected benefit obligation:**                                                                                      |                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                        |   |   |
| Defined Benefit Plan, Number of Employees Covered                                                                                | 1gac_DefinedBenefitPlanNumberofEmployeesCovered/ us-gaap_BalanceSheetLocationAxis= gac_OtherNoncurrentLiabilitiesWithOffsetToGoodwillMember/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember                          |                                                                                                                                                                                                        |   |   |
| Defined Benefit Plan, Benefit Obligation, Period Increase (Decrease)                                                             | $ 3,911us-gaap_DefinedBenefitPlanBenefitObligationPeriodIncreaseDecrease/ us-gaap_BalanceSheetLocationAxis= gac_OtherNoncurrentLiabilitiesWithOffsetToGoodwillMember/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember |                                                                                                                                                                                                        |   |   |

| X                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Benefit Plan, Number of Employees Covered
+ References
No definition available.
+ Details**Name:**gac_DefinedBenefitPlanNumberofEmployeesCovered**Namespace Prefix:**gac_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same.
+ References
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| - Definition
The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) related to change in benefit obligation resulting from changes in actuarial assumptions, for example, but not limited to, interest, mortality, employee turnover, salary, and temporary deviation from the substantive plan.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date.  2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net increase or decrease in the plan benefit obligation during the reporting period.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of increase in the benefit obligation attributed to business combinations.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The increase in the fair value of plan assets from contributions made by the employer.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of increase or decrease due to a change in the terms of an existing plan or the initiation of a new plan. A plan amendment may increase or decrease benefits, including those attributed to years of service already rendered.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Glossary Plan Amendment -URI http://asc.fasb.org/extlink&oid=6520743Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Plan Amendment -URI http://asc.fasb.org/extlink&oid=6520782Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(7) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.
+ References
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