Revenue from Contracts with Customers - Adjustments to Previously Reported Consolidated Financial Statements from the Adoption of Accounting Pronouncements (Details) - USD ($) $ 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
Assets
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
Liabilities
Advanced billings and customer deposits	7,415					7,415		6,851
Deferred income taxes	120,846	114,647	118,662		114,647	120,846		121,231	100,879
Other long-term liabilities	14,567					14,567		14,093	15,293
Retained earnings	385,045	369,511	359,885		369,511	385,045		353,302	297,205
Operating revenues	158,731			$ 151,782		469,370	$ 458,920
Selling, general and administrative	27,452			42,199		86,117	125,374
Calculated under Revenue Guidance in Effect before Topic 606
Assets
Prepaid expenses and other	27,765					27,765			17,111
Deferred charges and other assets, net	17,496					17,496			13,690
Liabilities
Advanced billings and customer deposits	23,744					23,744			21,153
Deferred income taxes	97,029					97,029			100,879
Other long-term liabilities	15,759					15,759			15,293
Retained earnings	319,496					319,496			$ 297,205
Selling, general and administrative	44,164					132,711
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Assets
Prepaid expenses and other	35,618					35,618		29,876
Deferred charges and other assets, net	36,227					36,227		31,071
Liabilities
Advanced billings and customer deposits	(16,329)					(16,329)		(14,302)
Deferred income taxes	23,817					23,817		20,352
Other long-term liabilities	(1,192)					(1,192)		(1,200)
Retained earnings	65,549					65,549		$ 56,097
Selling, general and administrative	(16,712)					(46,594)
Service revenue and other
Liabilities
Operating revenues	142,768	$ 140,492	$ 136,559	149,788	$ 277,051	419,819	450,617
Service revenue and other | Calculated under Revenue Guidance in Effect before Topic 606
Liabilities
Operating revenues	161,076					471,155
Service revenue and other | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Liabilities
Operating revenues	(18,308)					(51,336)
Equipment revenue
Liabilities
Operating revenues	15,963			1,994		49,551	8,303
Equipment revenue | Calculated under Revenue Guidance in Effect before Topic 606
Liabilities
Operating revenues	2,178					6,036
Equipment revenue | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Liabilities
Operating revenues	13,785					43,515
Cost of services
Liabilities
Cost of services	47,886			48,552		146,362	145,744
Cost of services | Calculated under Revenue Guidance in Effect before Topic 606
Liabilities
Cost of services	48,001					146,199
Cost of services | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Liabilities
Cost of services	(115)					163
Cost of goods sold
Liabilities
Cost of services	15,036			$ 7,282		46,007	$ 17,232
Cost of goods sold | Calculated under Revenue Guidance in Effect before Topic 606
Liabilities
Cost of services	7,870					20,316
Cost of goods sold | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Liabilities
Cost of services	$ 7,166					$ 25,691