Quarterly financial information (unaudited): (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Service revenue									$ 589,797	$ 568,103	$ 546,159
Network operations, including equity-based compensation expense									226,337	219,157	219,801
Operating income									119,233	106,922	100,257
Net income (loss)									$ 48,185	$ 6,216	$ 37,520
Net income (loss) per common share - basic									$ 1.04	$ 0.14	$ 0.82
Net income (loss) per common share - diluted									$ 1.03	$ 0.13	$ 0.81
Weighted-average number of common shares-basic									46,419,180	45,947,772	45,542,315
Weighted-average number of common shares-diluted									46,963,920	46,668,198	46,080,395
Unrealized foreign exchange gain (loss) on 2024 Euro Notes									$ 32,522	$ (36,997)	$ 2,271
Unrealized foreign currency exchange gain (loss)	$ 8,800	$ 10,200	$ (5,300)	$ 18,900	$ (19,200)	$ (17,300)	$ (3,400)	$ 2,900	$ 32,522	(36,997)	$ 2,271
Realized foreign exchange gain on 2024 Euro Notes			2,500							$ 2,533
Loss on debt extinguishment and redemption - 2021 Notes			10,800	3,900
Gain on lease termination	7,400
Unaudited
Service revenue	147,208	147,927	147,879	146,777	143,901	142,302	140,990	140,915
Network operations, including equity-based compensation expense	56,418	56,645	56,180	57,092	54,829	54,519	53,886	55,921
Operating income	36,165	28,556	28,211	26,291	27,384	26,036	27,574	25,850
Net income (loss)	$ 18,507	$ 13,320	$ (2,493)	$ 18,851	$ (6,620)	$ (6,555)	$ 8,564	$ 9,227
Net income (loss) per common share - basic	$ 0.40	$ 0.29	$ (0.05)	$ 0.41	$ (0.14)	$ (0.11)	$ 0.18	$ 0.20
Net income (loss) per common share - diluted	$ 0.39	$ 0.28	$ (0.05)	$ 0.41
Net income per common share-basic and diluted					$ (0.14)	$ (0.11)	$ 0.19	$ 0.20
Weighted-average number of common shares-basic	46,420,168	46,293,524	46,229,603	46,067,096	45,904,943	45,815,718	45,754,880	45,658,565
Weighted-average number of common shares-diluted	46,992,639	46,866,929	46,229,603	46,507,258	45,904,943	45,815,718	46,686,665	46,391,066