| **Fair Value Measurements - Recurring not carried at FV (Details 5) - USD ($) $ in Thousands**         | 9 Months Ended |             |
|--------------------------------------------------------------------------------------------------------|----------------|-------------|
||
| **Assets**                                                                                             |           |        |
| Mortgage loans on real estate                                                                          | $ 3,568,114    | $ 3,247,704 |
| Policy loans                                                                                           | 4,056,284      | 4,092,661   |
| **Liabilities**                                                                                        |           |        |
| Policyholders’ funds                                                                             | 254,712        | 299,577     |
| Commercial paper                                                                                       | $ 99,971       | 93,371      |
| Policyholders' funds, Notice required to change policyholders' funds interest crediting rate (in days) | 30 days        |        |
| Minimum                                                                                                |           |        |
| **Liabilities**                                                                                        |           |        |
| Limited partnership interests, Estimated liquidation period of underlying assets of the funds          | 1 year         |        |
| Maximum                                                                                                |           |        |
| **Liabilities**                                                                                        |           |        |
| Limited partnership interests, Estimated liquidation period of underlying assets of the funds          | 10 years       |        |
| Carrying amount                                                                                        |           |        |
| **Assets**                                                                                             |           |        |
| Mortgage loans on real estate                                                                          | $ 3,568,114    | 3,247,704   |
| Policy loans                                                                                           | 4,056,284      | 4,092,661   |
| Limited partnership interests                                                                          | 30,336         | 35,039      |
| Other investments                                                                                      | 14,293         | 14,596      |
| **Liabilities**                                                                                        |           |        |
| Annuity contract benefits without life contingencies                                                   | 12,254,743     | 11,104,721  |
| Policyholders’ funds                                                                             | 254,712        | 299,577     |
| Commercial paper                                                                                       | 99,971         | 93,371      |
| Notes payable                                                                                          | 534,299        | 532,575     |
| Estimated fair value                                                                                   |           |        |
| **Assets**                                                                                             |           |        |
| Mortgage loans on real estate                                                                          | 3,795,123      | 3,362,496   |
| Policy loans                                                                                           | 4,056,284      | 4,092,661   |
| Limited partnership interests                                                                          | 30,262         | 34,882      |
| Other investments                                                                                      | 44,723         | 44,723      |
| **Liabilities**                                                                                        |           |        |
| Annuity contract benefits without life contingencies                                                   | 12,357,043     | 10,839,205  |
| Policyholders’ funds                                                                             | 254,712        | 299,577     |
| Commercial paper                                                                                       | 99,971         | 93,371      |
| Notes payable                                                                                          | $ 515,795      | $ 563,633   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. It represents the annuity contract benefits without life contingencies of the entity valued at fair value by the entity.
+ References
No definition available.
+ Details**Name:**gac_AnnuityContractBenefitsWithoutLifeContingenciesFairValueDisclosure**Namespace Prefix:**gac_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Represents the liquidation period estimated by the entity of the underlying assets of the funds.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Notice required to change policyholders' funds interest crediting rate.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other investments not carried at fair value on a recurring basis not otherwise specified in the taxonomy.
+ References
No definition available.
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(a)(3)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.13(2)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of investments accounted under the cost method.
+ References
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| - References
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No definition available.
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before allowance of loans made to policy and contract holders using their life insurance contracts' cash value or separate accounts as collateral. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums over and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(e)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(a)(7)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of notes payable.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19,20,22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total liability as of the balance sheet date of amounts due policy holders, excluding future policy benefits and claims, including unpaid policy dividends, retrospective refunds, and undistributed earnings on participating business.
+ References
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