Quarterly financial information (unaudited): (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly financial information (unaudited):
Service revenue	$ 105,177	$ 103,017	$ 98,799	$ 97,242	$ 115,596	$ 113,057	$ 109,955	$ 108,291	$ 446,900	$ 404,234	$ 380,003
Network operations, including equity-based	45,836	45,182	42,412	41,079	50,089	48,827	47,872	47,277	194,066	174,510	159,893
Gains on equipment transactions	2,023	1,152	719	1,548	667	687	4,439	1,946	7,739	5,443	10,950
Gains on capital lease terminations		1,484		10,110						(11,643)
Losses on debt extinguishment and redemption				10,144					587	10,144
Operating income	16,174	15,519	10,810	10,487	14,795	16,063	17,511	15,675	64,042	53,036	53,900
Net income	$ 2,480	$ 3,161	$ 840	$ (1,585)	$ 3,892	$ 3,459	$ 4,224	$ 3,354	$ 14,929	$ 4,896	$ 797
Net (loss) per common share - basic and diluted (in dollars per share)	$ 0.06	$ 0.07	$ 0.02	$ (0.04)	$ 0.09	$ 0.08	$ 0.09	$ 0.08
Weighted-average number of common shares-basic	44,323,131	44,474,724	44,774,831	45,158,250	44,577,826	44,574,583	44,491,899	44,402,640	44,641,805	44,888,723	45,960,720
Weighted-average number of common shares-diluted	44,558,089	44,702,127	45,054,507	45,158,250	44,803,782	44,776,918	44,705,037	44,571,937	44,873,030	45,159,489	46,349,670