Summary of Significant Accounting Policies (Cumulative Effect of the Changes Made to the Statement of Income for the Adoption of the New Revenue Standard) (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
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	$ 549,386	$ 545,448	$ 580,752	$ 537,656	$ 506,121	$ 491,976	$ 508,940	$ 462,021	$ 2,213,242	$ 1,969,058	$ 1,775,423
Total cost of revenue									971,700	871,676	799,987
Gross profit	300,361	305,643	332,439	303,099	272,474	274,002	292,715	258,191	1,241,542	1,097,382	975,436
Total operating expense									750,207
Income from operations	$ 115,347	$ 117,350	$ 145,594	$ 113,044	$ 97,808	$ 100,413	$ 122,564	$ 92,243	491,335	413,028	350,239
Interest expense									(34,744)	(37,225)	(32,049)
Interest income									1,151	5,254	3,656
Income before provision for income taxes									457,742	381,057	321,846
Provision for income taxes									80,695	117,788	99,792
Net income									$ 377,047	$ 263,269	$ 222,054
Earnings per Share:
Basic (in USD per share)	$ 0.99	$ 1.07	$ 1.25	$ 1.02	$ 0.44	$ 0.81	$ 0.97	$ 0.78	$ 4.34	$ 3.00	$ 2.47
Diluted (in USD per share)	$ 0.98	$ 1.05	$ 1.23	$ 1.01	$ 0.43	$ 0.79	$ 0.95	$ 0.77	$ 4.26	$ 2.94	$ 2.44
Previous U.S. GAAP
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									$ 2,154,902
Total cost of revenue									936,302
Gross profit									1,218,600
Total operating expense									752,268
Income from operations									466,332
Interest expense									(34,744)
Interest income									2,198
Income before provision for income taxes									433,786
Provision for income taxes									74,865
Net income									358,921
Accounting Standards Update 2014-09 | Attributed to the New Revenue Standard
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									58,340
Total cost of revenue									35,398
Gross profit									22,942
Total operating expense									(2,061)
Income from operations									25,003
Interest expense									0
Interest income									(1,047)
Income before provision for income taxes									23,956
Provision for income taxes									5,830
Net income									$ 18,126