| **Business and Summary of Significant Accounting Principles (Changes in Issued Common Stock) (Details)In Thousands, unless otherwise specified** | 12 Months Ended                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Common Stock [Roll Forward]**                                                                                                                  |                                                                                                                                     |   |   |
| Shares issued upon stock option exercises                                                                                                        | 50us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised                                                                             |   |   |
| Class A Common Stock                                                                                                                             |                                                                                                                                     |   |   |
| **Common Stock [Roll Forward]**                                                                                                                  |                                                                                                                                     |   |   |
| Balance, Beginning                                                                                                                               | 37,299us-gaap_CommonStockSharesIssued/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember                                     | 38,534us-gaap_CommonStockSharesIssued/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember | 39,296us-gaap_CommonStockSharesIssued/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember |
| Class B shares converted to Class A                                                                                                              | 6us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember   | 4us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember | 2us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember |
| Shares issued upon stock option exercises                                                                                                        | 51us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember              | 87us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember | 320us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember |
| Share awards issued                                                                                                                              | 1,267us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember       | 680us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember | 731us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember |
| Shares repurchased and retired                                                                                                                   | (429)us-gaap_StockRepurchasedAndRetiredDuringPeriodShares/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember                 | (1,822)us-gaap_StockRepurchasedAndRetiredDuringPeriodShares/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember | (1,469)us-gaap_StockRepurchasedAndRetiredDuringPeriodShares/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember |
| Shares acquired to settle minimum statutory tax withholding requirements and subsequently retired                                                | (196)us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember         | (147)us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember | (337)us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember |
| Other                                                                                                                                            |                                                                                                                                     | (37)us-gaap_StockholdersEquityOtherShares/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember | (9)us-gaap_StockholdersEquityOtherShares/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember |
| Balance, Ending                                                                                                                                  | 37,998us-gaap_CommonStockSharesIssued/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember                                     | 37,299us-gaap_CommonStockSharesIssued/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember | 38,534us-gaap_CommonStockSharesIssued/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember |
| Class B Common Stock                                                                                                                             |                                                                                                                                     |   |   |
| **Common Stock [Roll Forward]**                                                                                                                  |                                                                                                                                     |   |   |
| Balance, Beginning                                                                                                                               | 3,165us-gaap_CommonStockSharesIssued/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember                                      | 3,169us-gaap_CommonStockSharesIssued/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember | 3,171us-gaap_CommonStockSharesIssued/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember |
| Class B shares converted to Class A                                                                                                              | (6)us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember | (4)us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember | (2)us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember |
| Shares issued upon stock option exercises                                                                                                        | 0us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember               | 0us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember | 0us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember |
| Share awards issued                                                                                                                              | 0us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember           | 0us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember | 0us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember |
| Shares repurchased and retired                                                                                                                   | 0us-gaap_StockRepurchasedAndRetiredDuringPeriodShares/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember                     | 0us-gaap_StockRepurchasedAndRetiredDuringPeriodShares/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember | 0us-gaap_StockRepurchasedAndRetiredDuringPeriodShares/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember |
| Shares acquired to settle minimum statutory tax withholding requirements and subsequently retired                                                | 0us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember             | 0us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember | 0us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember |
| Other                                                                                                                                            |                                                                                                                                     | 0us-gaap_StockholdersEquityOtherShares/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember | 0us-gaap_StockholdersEquityOtherShares/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember |
| Balance, Ending                                                                                                                                  | 3,159us-gaap_CommonStockSharesIssued/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember                                      | 3,165us-gaap_CommonStockSharesIssued/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember | 3,169us-gaap_CommonStockSharesIssued/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassBMember |

| X                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**gncma_CommonStockRollForward**Namespace Prefix:**gncma_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.
+ 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 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5
+ Details**Name:**us-gaap_CommonStockSharesIssued**Namespace Prefix:**us-gaap_**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
For net-share settlement of share-based awards when the employer settles employees' income tax withholding obligations, this element represents the number of shares the employees use to repay the employer.
+ References
No definition available.
+ Details**Name:**us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation**Namespace Prefix:**us-gaap_**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents movements in the number of shares included in the statement of changes in stockholders' equity which are not separately disclosed or provided for elsewhere in the taxonomy.
+ References
No definition available.
+ Details**Name:**us-gaap_StockholdersEquityOtherShares**Namespace Prefix:**us-gaap_**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of shares issued during the period as a result of the conversion of convertible securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29-30) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total number of shares issued during the period, including shares forfeited, as a result of Restricted Stock Awards.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770
+ Details**Name:**us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross**Namespace Prefix:**us-gaap_**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of share options (or share units) exercised during the current period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.28,29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5
+ Details**Name:**us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised**Namespace Prefix:**us-gaap_**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of shares that have been repurchased and retired during the period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5
+ Details**Name:**us-gaap_StockRepurchasedAndRetiredDuringPeriodShares**Namespace Prefix:**us-gaap_**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassBMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |