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
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Assets [Abstract]
Prepaid expenses and other	$ 64,163		$ 64,163		$ 53,688	$ 17,111
Deferred charges and other assets, net	34,021		34,021		29,797	13,690
Liabilities [Abstract]
Advanced billings and customer deposits	6,668		6,668		6,851	21,153
Deferred income taxes	111,125		111,125		119,030	100,879
Other long-term liabilities	15,080		15,080		14,093	15,293
Retained earnings	359,893		359,893		347,240	297,205
Operating revenues	154,030	$ 153,258	305,762	$ 307,138
Operating expenses:	49,134	48,416	98,476	97,193
Cost of goods sold	15,166	4,965	30,971	9,949
Selling, general and administrative	29,915	43,022	58,665	83,175
Calculated under Revenue Guidance in Effect before Topic 606
Assets [Abstract]
Prepaid expenses and other	26,215		26,215			17,111
Deferred charges and other assets, net	18,094		18,094			13,690
Liabilities [Abstract]
Advanced billings and customer deposits	22,704		22,704			21,153
Deferred income taxes	92,190		92,190			100,879
Other long-term liabilities	16,259		16,259			15,293
Retained earnings	307,738		307,738			$ 297,205
Operating expenses:	48,999		98,198
Cost of goods sold	6,328		12,446
Selling, general and administrative	45,579		88,547
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Assets [Abstract]
Prepaid expenses and other	37,948		37,948		36,577
Deferred charges and other assets, net	15,927		15,927		16,107
Liabilities [Abstract]
Advanced billings and customer deposits	(16,036)		(16,036)		(14,302)
Deferred income taxes	18,935		18,935		18,151
Other long-term liabilities	(1,179)		(1,179)		(1,200)
Retained earnings	52,155		52,155		$ 50,035
Operating expenses:	135		278
Cost of goods sold	8,838		18,525
Selling, general and administrative	(15,664)		(29,882)
Service revenues and other
Liabilities [Abstract]
Operating revenues	138,021	150,308	272,174	300,829
Service revenues and other | Calculated under Revenue Guidance in Effect before Topic 606
Liabilities [Abstract]
Operating revenues	156,267		310,079
Service revenues and other | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Liabilities [Abstract]
Operating revenues	(18,246)		(37,905)
Equipment revenues
Liabilities [Abstract]
Operating revenues	16,009	$ 2,950	33,588	$ 6,309
Equipment revenues | Calculated under Revenue Guidance in Effect before Topic 606
Liabilities [Abstract]
Operating revenues	1,799		3,858
Equipment revenues | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Liabilities [Abstract]
Operating revenues	$ 14,210		$ 29,730