Revenues (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts Receivable, Net, Current	$ 174,411		$ 234,195	$ 234,195
Deferred revenue, current	(113,940)		(93,656)	(95,255)
Deferred revenue, non-current	8,018		47	47
Retained earnings	723,562		$ 724,661	$ 723,062
Total revenues	280,623	$ 254,669
Costs and Expenses	261,752
Net Income (Loss) Attributable to Parent	$ 18,871	$ 25,820
Diluted	$ 0.38	$ 0.52
Deferred Revenue, Revenue Recognized	$ 38,900
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Deferred Revenue	23,890
Accounts Receivable, net [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Deferred Revenue, current [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	(1,599)
Deferred Revenue, noncurrent [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Retained Earnings [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	1,599
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts Receivable, Net, Current	0
Deferred revenue, current	(416)
Deferred revenue, non-current	0
Retained earnings	416
Total revenues	2,015
Costs and Expenses	0
Net Income (Loss) Attributable to Parent	$ 2,015
Diluted	$ 0.04
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts Receivable, Net, Current	$ 174,411
Deferred revenue, current	(113,524)
Deferred revenue, non-current	8,018
Retained earnings	723,978
Total revenues	282,638
Costs and Expenses	261,752
Net Income (Loss) Attributable to Parent	$ 20,886
Diluted	$ 0.42