Significant Accounting Policies (Impact of Adoption of ASU 2014-09) (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
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 4,169,019				$ 3,509,484				$ 4,169,019	$ 3,509,484
Deferred income tax expense									(13,156)	94,158		$ (21,654)
Total net revenues	801,870	$ 732,476	$ 742,365	$ 697,192	777,857	$ 690,925	$ 693,227	$ 662,677	2,973,903	2,824,686	[1],[2]	2,736,080	[1],[2]
Total cost of net revenues									433,803	439,646		404,889
Gross profit	689,019	628,559	633,616	588,906	655,918	584,988	583,915	560,219	2,540,100	2,385,040		2,331,191
Total operating expenses									1,862,140	1,814,043		1,771,027
Income from operations	202,471	164,779	145,147	165,563					677,960	570,997		560,164
Net income	$ 165,718	$ 158,857	$ 106,833	$ 144,259	$ (283,889)	$ 126,720	$ 108,829	$ 27,621	$ 575,667	$ (20,719)		$ 536,112
Basic earnings earnings per share (USD per share)	$ 1.24	$ 1.18	$ 0.79	$ 1.04	$ (1.93)	$ 0.84	$ 0.72	$ 0.18	$ 4.23	$ (0.13)		$ 3.46
Diluted earnings per share (USD per share)	$ 1.15	$ 1.08	$ 0.73	$ 0.99	$ (1.93)	$ 0.82	$ 0.70	$ 0.17	$ 3.94	$ (0.13)		$ 3.41
Prepaid expenses and other current assets	$ 174,195				$ 147,330				$ 174,195	$ 147,330
Other assets	124,578				52,685				124,578	52,685
Deferred tax assets, net	136,998				152,362				136,998	152,362
Total assets	5,136,049				5,820,176				5,136,049	5,820,176
Other liabilities	148,499				121,936				148,499	121,936
Current portion of deferred revenues	1,345,243				1,308,474				1,345,243	1,308,474				$ 1,250,000
Long-term portion of deferred revenues	489,329				555,769				489,329	555,769				512,800
Total liabilities	4,576,420								4,576,420
Deferred tax liabilities	49,187				$ 10,518				49,187	$ 10,518
Balances without adoption of ASC 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	4,009,637								4,009,637
Total net revenues									2,966,848
Total cost of net revenues									431,974
Gross profit									2,534,874
Total operating expenses									1,890,692
Income from operations									644,182
Net income									$ 548,430
Basic earnings earnings per share (USD per share)									$ 4.03
Diluted earnings per share (USD per share)									$ 3.76
Prepaid expenses and other current assets	147,554								$ 147,554
Other assets	52,732								52,732
Deferred tax assets, net	169,064								169,064
Total assets	5,069,628								5,069,628
Other liabilities	135,430								135,430
Current portion of deferred revenues	1,413,839								1,413,839
Long-term portion of deferred revenues	526,763								526,763
Total liabilities	4,669,381								4,669,381
Deferred tax liabilities	56,600								56,600
Accounting Standards Update 2014-09 | Effect of Change Higher/(Lower)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	159,382								159,382					130,700
Relative stand-alone selling price									99,900
Contract assets														$ 7,300
Sales commissions and fees									66,400
Deferred income tax expense		$ 42,900
Total net revenues									7,055
Total cost of net revenues									1,829
Gross profit									5,226
Total operating expenses									(28,552)
Income from operations									33,778
Net income									$ 27,237
Basic earnings earnings per share (USD per share)									$ 0.20
Diluted earnings per share (USD per share)									$ 0.18
Prepaid expenses and other current assets	26,641								$ 26,641
Other assets	71,846								71,846
Deferred tax assets, net	(32,066)								(32,066)
Total assets	66,421								66,421
Other liabilities	13,069								13,069
Current portion of deferred revenues	(68,596)								(68,596)
Long-term portion of deferred revenues	(37,434)								(37,434)
Total liabilities	(92,961)								(92,961)
Prepaid expenses and other current assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract acquisition costs	41,000								41,000
Prepaid expenses and other current assets | Balances without adoption of ASC 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract acquisition costs	14,200								14,200
Other assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract acquisition costs	68,200								68,200
Other liabilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax liabilities	$ 54,700								$ 54,700

[1] As noted above, prior period amounts have not been adjusted under the modified retrospective method of adoption of therevenue recognition standard. See Note 2 for further information regarding the Company’s adoption of the revenuerecognition standard.

[2] Prior period amounts have not been adjusted under the modified retrospective method of adoption of the revenue recognition standard. See Note 2 for further information regarding the Company’s adoption of the revenue recognition standard.