Basis of Presentation - Immaterial Prior Period Adjustment (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended	9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Prepaid expenses and other	$ 63,383	$ 59,407	$ 58,459		$ 59,407	$ 63,383		$ 46,987	$ 17,111
Deferred charges and other assets, net	53,723	51,917	50,344		51,917	53,723		44,761	13,690
Deferred income taxes	120,846	114,647	118,662		114,647	120,846		121,231	100,879
Retained earnings	385,045	369,511	359,885		369,511	385,045		353,302	$ 297,205
Service revenue and other	158,731			$ 151,782		469,370	$ 458,920
Income tax expense (benefit)	4,848	3,531	1,828	(2,677)	5,359	10,207	(1,830)
Net income (loss)	$ 15,534	$ 9,626	$ 6,583	$ 3,534	$ 16,209	$ 31,743	$ 5,795
Earnings per share - basic (in usd per share)	$ 0.31	$ 0.19	$ 0.13	$ 0.07	$ 0.33	$ 0.64	$ 0.12
Earnings per share - diluted (in usd per share)	$ 0.31	$ 0.19	$ 0.13	$ 0.07	$ 0.32	$ 0.63	$ 0.12
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 expense (benefit)		2,862	1,176		4,038
Net income (loss)		$ 7,824	$ 4,829		$ 12,653
Earnings per share - basic (in usd per share)		$ 0.16	$ 0.10		$ 0.26
Earnings per share - diluted (in usd per share)		$ 0.16	$ 0.10		$ 0.25
Service revenue and other
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Service revenue and other	$ 142,768	$ 140,492	$ 136,559	$ 149,788	$ 277,051	$ 419,819	$ 450,617
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
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 expense (benefit)		669	652		1,321
Net income (loss)		$ 1,802	$ 1,754		$ 3,556
Earnings per share - basic (in usd per share)		$ 0.03	$ 0.03		$ 0.07
Earnings per share - diluted (in usd per share)		$ 0.03	$ 0.03		$ 0.07
Immaterial Error Correction | Service revenue and other | Correction of Error
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Service revenue and other		$ 2,471	$ 2,406		$ 4,877