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								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	$ 160,997	$ 155,152	$ 158,914	$ 158,843	$ 161,484	$ 158,731	$ 156,501	$ 154,138	$ 633,906	$ 630,854	$ 611,991
Selling, general and administrative									112,540	113,222	165,937
Assets
Prepaid expenses and other	57,805				60,162				57,805	60,162		$ 48,582
Deferred charges and other assets	53,352				49,891				53,352	49,891
Liabilities
Advanced billing and customer deposits					7,919					7,919
Deferred income taxes	136,451				127,453				136,451	127,453
Other long-term liabilities	16,990				14,364				16,990	14,364		$ 16,008
Retained earnings	$ 427,004				386,511				427,004	386,511
Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Selling, general and administrative										175,753
Assets
Prepaid expenses and other					22,204					22,204
Deferred charges and other assets					12,083					12,083
Liabilities
Advanced billing and customer deposits					24,414					24,414
Deferred income taxes					103,404					103,404
Other long-term liabilities					15,550					15,550
Retained earnings					319,926					319,926
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Selling, general and administrative										(62,531)
Assets
Prepaid expenses and other					37,958					37,958
Deferred charges and other assets					37,808					37,808
Liabilities
Advanced billing and customer deposits					(16,495)					(16,495)
Deferred income taxes					24,049					24,049
Other long-term liabilities					(1,186)					(1,186)
Retained earnings					$ 66,585					66,585
Service revenue and other
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									565,063	562,456	601,673
Service revenue and other | Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue										632,340
Service revenue and other | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue										(69,884)
Equipment revenue
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									$ 68,843	68,398	$ 10,318
Equipment revenue | Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue										8,298
Equipment revenue | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue										60,100
Cost of services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating expenses										194,022
Cost of services | Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating expenses										193,860
Cost of services | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating expenses										162
Cost of goods sold
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating expenses										63,959
Cost of goods sold | Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating expenses										28,377
Cost of goods sold | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating expenses										$ 35,582