Quarterly financial information (unaudited): (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly financial information (unaudited):
Service revenue	$ 105,177	$ 103,017	$ 98,799	$ 97,242	$ 96,749	$ 95,691	$ 94,623	$ 92,937	$ 404,234	$ 380,003	$ 347,979
Network operations, including equity-based compensation expense, respectively, exclusive of amounts shown separately)	45,836	45,182	42,412	41,079	41,046	40,407	39,605	38,836	174,510	159,893	150,225
Gains on equipment transactions	2,023	1,152	719	1,548	2,810	3,114	2,731	2,295	5,443	10,950
Gain on Lease termination		1,484		10,110					11,643
Loss on Debt extinguishment and redemption				(10,144)					10,144
Operating income	16,174	15,519	10,810	10,487	13,066	13,614	14,309	12,907	53,036	53,900	46,154
Net income	$ 2,480	$ 3,161	$ 840	$ (1,585)	$ (352)	$ (184)	$ 1,208	$ 125	$ 4,896	$ 797	$ 56,689
Net income (loss) per common share-basic (in dollars per share)									$ 0.11	$ 0.02	$ 1.22
Net income (loss) per common share-diluted (in dollars per share)	$ 0.06	$ 0.07	$ 0.02	$ (0.04)	$ (0.01)	$ 0.00	$ 0.03	$ 0.00	$ 0.11	$ 0.02	$ 1.21
Weighted-average number of common shares-basic	44,323,131	44,474,724	44,774,831	45,158,250	45,229,125,000	45,629,079,000	45,897,449,000	46,409,735,000	44,888,723	45,960,720	46,286,735
Weighted-average number of common shares-diluted	44,558,089	44,702,127	45,054,507	45,158,250	45,229,125	45,629,079	46,294,966	46,907,360	45,159,489	46,349,670	46,996,904