Long Term Debt (Schedule of Redemption Prices) (Details)	12 Months Ended
Dec. 31, 2012
2021 Notes [Member]
Debt Instrument [Line Items]
2016	103.375%
2017	102.25%
2018	101.125%
2019 and thereafter	100.00%
2019 Notes [Member]
Debt Instrument [Line Items]
2014	104.313%
2015	102.875%
2016	101.438%
2017 and thereafter	100.00%

Redemption price in percentage in 2018 after the redemption date.

No definition available.

Name:	gncma_RedemptinPriceInTwentyEighteen
Namespace Prefix:	gncma_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Redemption price in percentage in 2015 after the redemption date.

No definition available.

Name:	gncma_RedemptinPriceInTwentyFifteen
Namespace Prefix:	gncma_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Redemption price in percentage in 2014 after the redemption date.

No definition available.

Name:	gncma_RedemptinPriceInTwentyFourteen
Namespace Prefix:	gncma_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Redemption price in percentage in 2019 and thereafter after the redemption date.

No definition available.

Name:	gncma_RedemptinPriceInTwentyNineteenAndThereafter
Namespace Prefix:	gncma_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Redemption price in percentage in 2017 after the redemption date.

No definition available.

Name:	gncma_RedemptinPriceInTwentySeventeen
Namespace Prefix:	gncma_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Redemption price in percentage in 2017 and thereafter after the redemption date.

No definition available.

Name:	gncma_RedemptinPriceInTwentySeventeenAndThereafter
Namespace Prefix:	gncma_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Redemption price in percentage in 2016 after the redemption date.

No definition available.

Name:	gncma_RedemptinPriceInTwentySixteen
Namespace Prefix:	gncma_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration