Annual report pursuant to Section 13 and 15(d)

Business and Summary of Significant Accounting Principles (Weighted Average Shares Outstanding Which Are Anti-Dilutive) (Details)

v2.4.1.9
Business and Summary of Significant Accounting Principles (Weighted Average Shares Outstanding Which Are Anti-Dilutive) (Details)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Equity Option      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Shares associated with anti-dilutive 29us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_StockOptionMember
86us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_StockOptionMember
88us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_StockOptionMember
Contingent Shares      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Shares associated with anti-dilutive 26us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= gncma_ContingentSharesMember
90us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= gncma_ContingentSharesMember
0us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= gncma_ContingentSharesMember