| **CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (USD $)In Thousands, unless otherwise specified** | Total     | Class A Shares held in Treasury [Member] | Paid-in Capital [Member] | Retained Earnings [Member] | Noncontrolling Interests [Member] | Common Stock - Class A [Member] | Common Stock - Class B [Member] |
|-----------------------------------------------------------------------------------------------------|-----------|------------------------------------------|--------------------------|----------------------------|-----------------------------------|---------------------------------|---------------------------------|
| Beginning balances, total stockholders' equity at Dec. 31, 2012                                     | $ 189,436 | $ (1,617)                                | $ 25,832                 | $ 107,584                  | $ 32,258                          | $ 22,703                        | $ 2,676                         |
| **Increase (Decrease) in Stockholders' Equity [Roll Forward]**                                      |      |                                     |                     |                       |                              |                            |                            |
| Net income (loss)                                                                                   | 7,190     |                                     |                     | 7,424                      | (234)                             |                            |                            |
| Common stock repurchases and retirements                                                            | (13,062)  | 130                                      |                     |                       |                              | (13,192)                        |                            |
| Shares issued under stock option plan                                                               | 314       |                                     |                     |                       |                              | 314                             |                            |
| Issuance of restricted stock awards                                                                 | 0         |                                     | (1,005)                  |                       |                              | 1,005                           |                            |
| Share-based compensation expense                                                                    | 3,094     |                                     | 3,094                    |                       |                              |                            |                            |
| Issuance of treasury shares related to deferred compensation payment                                | 621       | 621                                      |                     |                       |                              |                            |                            |
| Other                                                                                               | 0         |                                     |                     |                       |                              | 2                               | (2)                             |
| Ending balances, total stockholders' equity at Jun. 30, 2013                                        | 187,593   | (866)                                    | 27,921                   | 115,008                    | 32,024                            | 10,832                          | 2,674                           |
| Beginning balances, total stockholders' equity at Dec. 31, 2013                                     | 457,354   | (866)                                    | 26,880                   | 116,990                    | 300,210                           | 11,467                          | 2,673                           |
| **Increase (Decrease) in Stockholders' Equity [Roll Forward]**                                      |      |                                     |                     |                       |                              |                            |                            |
| Net income (loss)                                                                                   | 30,810    |                                     |                     | 10,276                     | 20,534                            |                            |                            |
| Common stock repurchases and retirements                                                            | (536)     |                                     |                     |                       |                              | (536)                           |                            |
| Shares issued under stock option plan                                                               | 176       |                                     |                     |                       |                              | 176                             |                            |
| Issuance of restricted stock awards                                                                 | 0         |                                     | (1,691)                  |                       |                              | 1,691                           |                            |
| Share-based compensation expense                                                                    | 3,973     |                                     | 3,973                    |                       |                              |                            |                            |
| Issuance of treasury shares related to deferred compensation payment                                | 715       | 617                                      | 98                       |                       |                              |                            |                            |
| Distribution to non-controlling interest                                                            | (25,000)  |                                     |                     |                       | (25,000)                          |                            |                            |
| Adjustment to investment by non-controlling interest                                                | (2,131)   |                                     |                     |                       | (2,131)                           |                            |                            |
| Other                                                                                               | 100       |                                     |                     |                       | 100                               | 3                               | (3)                             |
| Ending balances, total stockholders' equity at Jun. 30, 2014                                        | $ 465,461 | $ (249)                                  | $ 29,260                 | $ 127,266                  | $ 293,713                         | $ 12,801                        | $ 2,670                         |

| X                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Noncontrolling Interest, Decrease from Adjustment to Investment
+ References
No definition available.
+ Details**Name:**gncma_NoncontrollingInterestDecreasefromAdjustmenttoInvestment**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the amount of recognized equity-based compensation during the period, that is, the amount recognized as expense in the income statement (or as asset if compensation is capitalized). Alternate captions include the words "stock-based compensation".
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 35 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415241&loc=d3e4534-113899Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 20 -Section 55 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=32706628&loc=d3e11149-113907Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 20 -Section 55 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=32706628&loc=d3e11178-113907
+ Details**Name:**us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_IncreaseDecreaseInStockholdersEquityRollForward**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Decrease in noncontrolling interest balance from payment of dividends or other distributions by the non-wholly owned subsidiary or partially owned entity, included in the consolidation of the parent entity, to the noncontrolling interest holders.
+ References
Reference 1: 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_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 55 -Paragraph 4K -URI http://asc.fasb.org/extlink&oid=31814832&loc=SL4591552-111686Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 19 -URI http://asc.fasb.org/extlink&oid=7656940&loc=SL4569616-111683Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=18733093&loc=SL4573702-111684Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 55 -Paragraph 4J -URI http://asc.fasb.org/extlink&oid=31814832&loc=SL4591551-111686
+ Details**Name:**us-gaap_ProfitLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of stockholders' equity (deficit), net of receivables from officers, directors, owners, and affiliates of the entity, attributable to both the parent and noncontrolling interests. Amount excludes temporary equity. Alternate caption for the concept is permanent equity.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=7656940&loc=SL4568447-111683Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 16 -URI http://asc.fasb.org/extlink&oid=7656940&loc=SL4568740-111683Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 55 -Paragraph 4I -URI http://asc.fasb.org/extlink&oid=31814832&loc=SL4590271-111686
+ Details**Name:**us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents movements 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_StockholdersEquityOther**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Value of stock related to Restricted Stock Awards issued during the period, net of the stock value of such awards forfeited.
+ 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_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Value of stock issued as a result of the exercise of stock options.
+ 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 210.5-02.29-31) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30, 31 -Article 5
+ Details**Name:**us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Equity impact of the value of treasury stock (units) reissued during the period, excluding reissuance of shares (units) held in treasury used to satisfy equity-based compensation obligations exercised by the holders of such rights. Upon reissuance of shares (units) from treasury, either the common or preferred stock (unit) reissued is outstanding.
+ 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 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655
+ Details**Name:**us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Equity impact of the value of stock that has been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital).
+ 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, 31 -Article 5
+ Details**Name:**us-gaap_StockRepurchasedAndRetiredDuringPeriodValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |