Revenues - Cumulative Effect Transition (Details) - USD ($) $ / shares in Units, $ in Thousands	3 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	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net of allowances of $10,422 and $8,701, respectively	$ 221,615				$ 234,195				$ 221,615	$ 234,195		$ 234,195
Deferred revenue, current	(127,568)				(95,255)				(127,568)	(95,255)		(93,656)
Deferred revenue, noncurrent	13,200				47				13,200	47		47
Retained earnings	769,575				723,062				769,575	723,062		$ 724,661
Total revenues	346,059	$ 292,724	$ 287,889	$ 280,623	316,380	$ 273,616	$ 273,174	$ 254,669	1,207,295	1,117,838	$ 874,255
Costs and Expenses									1,078,608
Net Income (Loss) Attributable to Parent	$ 50,614	$ 30,723	$ 28,479	$ 18,871	$ 49,871	$ 32,358	$ 31,376	$ 25,820	$ 128,687	$ 139,425	$ 152,439
Earnings Per Share, Diluted	$ 1.03	$ 0.61	$ 0.57	$ 0.38	$ 1.02	$ 0.66	$ 0.63	$ 0.52	$ 2.59	$ 2.83	$ 3.13
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 1,599								$ 1,599
Deferred Revenue, Revenue Recognized									80,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Deferred Revenue	37,113				$ 11,853				37,113	$ 11,853	$ 6,904
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								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)								(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								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								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 of allowances of $10,422 and $8,701, respectively	0								0
Deferred revenue, current	(5,455)								(5,455)
Deferred revenue, noncurrent	0								0
Retained earnings	5,455								5,455
Total revenues									7,054
Costs and Expenses									0
Net Income (Loss) Attributable to Parent									$ 7,054
Earnings Per Share, Diluted									$ 0.14
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net of allowances of $10,422 and $8,701, respectively	221,615								$ 221,615
Deferred revenue, current	(122,113)								(122,113)
Deferred revenue, noncurrent	13,200								13,200
Retained earnings	$ 775,030								775,030
Total revenues									1,214,349
Costs and Expenses									1,078,608
Net Income (Loss) Attributable to Parent									$ 135,741
Earnings Per Share, Diluted									$ 2.73