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	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
Assets
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
Liabilities
Advanced billings and customer deposits	7,919									7,919			6,851
Deferred income taxes	127,453		114,647	118,662	100,879				114,647	127,453	100,879		121,231
Other long-term liabilities	14,364				15,293					14,364	15,293		14,093
Retained earnings	386,511		369,511	359,885	297,205				369,511	386,511	297,205		353,302
Total operating revenue	161,484	$ 158,731	156,501	154,138	151,617	$ 152,382	$ 153,867	$ 154,125		630,854	611,991	$ 535,288
Selling, general and administrative										113,222	165,937	133,325
Calculated under Revenue Guidance in Effect before Topic 606
Assets
Prepaid expenses and other	22,204				17,111					22,204	17,111
Deferred charges and other assets, net	12,083				13,690					12,083	13,690
Liabilities
Advanced billings and customer deposits	24,414				21,153					24,414	21,153
Deferred income taxes	103,404				100,879					103,404	100,879
Other long-term liabilities	15,550				15,293					15,550	15,293
Retained earnings	319,926				$ 297,205					319,926	297,205
Selling, general and administrative										175,753
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Assets
Prepaid expenses and other	37,958									37,958			29,876
Deferred charges and other assets, net	37,808									37,808			31,071
Liabilities
Advanced billings and customer deposits	(16,495)									(16,495)			(14,302)
Deferred income taxes	24,049									24,049			20,352
Other long-term liabilities	(1,186)									(1,186)			(1,200)
Retained earnings	$ 66,585									66,585			$ 56,097
Selling, general and administrative										(62,531)
Service revenue and other
Liabilities
Total operating revenue			$ 140,492	$ 136,559					$ 277,051	562,456	601,673	523,748
Service revenue and other | Calculated under Revenue Guidance in Effect before Topic 606
Liabilities
Total operating revenue										632,340
Service revenue and other | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Liabilities
Total operating revenue										(69,884)
Equipment revenue
Liabilities
Total operating revenue										68,398	10,318	11,540
Equipment revenue | Calculated under Revenue Guidance in Effect before Topic 606
Liabilities
Total operating revenue										8,298
Equipment revenue | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Liabilities
Total operating revenue										60,100
Cost of services
Liabilities
Cost of services										194,022	188,721	163,969
Cost of services | Calculated under Revenue Guidance in Effect before Topic 606
Liabilities
Cost of services										193,860
Cost of services | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Liabilities
Cost of services										162
Cost of goods sold
Liabilities
Cost of services										63,959	$ 22,786	$ 29,551
Cost of goods sold | Calculated under Revenue Guidance in Effect before Topic 606
Liabilities
Cost of services										28,377
Cost of goods sold | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Liabilities
Cost of services										$ 35,582