Quarterly Results (unaudited) Correction of Prior Period Adjustment (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Service revenue and other	$ 161,484	$ 158,731	$ 156,501	$ 154,138	$ 151,617	$ 152,382	$ 153,867	$ 154,125		$ 630,854	$ 611,991	$ 535,288
Prepaid expenses and other	60,162		59,407	58,459	17,111				$ 59,407	60,162	17,111		$ 46,987
Deferred charges and other assets, net	49,891		51,917	50,344	13,690				51,917	49,891	13,690		44,761
Deferred income taxes	127,453		114,647	118,662	100,879				114,647	127,453	100,879		121,231
Retained earnings	386,511		369,511	359,885	297,205				369,511	386,511	297,205		353,302
Income tax (benefit) expense			3,531	1,828					5,359	15,517	(53,133)	2,840
Net income (loss)	$ 14,852	$ 15,534	$ 9,626	$ 6,583	$ 60,595	$ 3,534	$ (80)	$ 2,341	$ 16,209	$ 46,595	$ 66,390	$ (895)
Basic net income (loss) per share (in dollars per share)	$ 0.31	$ 0.31	$ 0.19	$ 0.13	$ 1.23	$ 0.07	$ 0.00	$ 0.05	$ 0.33	$ 0.94	$ 1.35	$ (0.02)
Diluted income (loss) per share (in dollars per share)	$ 0.30	$ 0.31	$ 0.19	$ 0.13	$ 1.21	$ 0.07	$ 0.00	$ 0.05	$ 0.32	$ 0.93	$ 1.33	$ (0.02)
As Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Prepaid expenses and other			$ 64,163	$ 64,200					$ 64,163				53,688
Deferred charges and other assets, net			34,021	33,934					34,021				29,797
Deferred income taxes			111,125	115,809					111,125				119,030
Retained earnings			359,893	352,069					359,893				347,240
Income tax (benefit) expense			2,862	1,176					4,038
Net income (loss)			$ 7,824	$ 4,829					$ 12,653
Basic net income (loss) per share (in dollars per share)			$ 0.16	$ 0.10					$ 0.26
Diluted income (loss) per share (in dollars per share)			$ 0.16	$ 0.10					$ 0.25
Immaterial error correction | Correction of Error
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Prepaid expenses and other			$ (4,756)	$ (5,741)					$ (4,756)				(6,701)
Deferred charges and other assets, net			17,896	16,410					17,896				14,964
Deferred income taxes			3,522	2,853					3,522				2,201
Retained earnings			9,618	7,816					9,618				$ 6,062
Income tax (benefit) expense			669	652					1,321
Net income (loss)			$ 1,802	$ 1,754					$ 3,556
Basic net income (loss) per share (in dollars per share)			$ 0.03	$ 0.03					$ 0.07
Diluted income (loss) per share (in dollars per share)			$ 0.03	$ 0.03					$ 0.07
Service revenue and other
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Service revenue and other			$ 140,492	$ 136,559					$ 277,051	$ 562,456	$ 601,673	$ 523,748
Service revenue and other | As Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Service revenue and other			138,021	134,153					272,174
Service revenue and other | Immaterial error correction | Correction of Error
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Service revenue and other			$ 2,471	$ 2,406					$ 4,877