Segments (Narrative) (Details) (Fiber optic cable systems [Member])	6 Months Ended
Jun. 30, 2014
Fiber optic cable systems [Member]
Segment Reporting Information [Line Items]
Percentage of long-lived assets not in USA	82.00%

Segment Reporting Information, Percentage of Long-Lived Assets Not Located in Country of Origin

No definition available.

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