Significant Accounting Policies - Impact of Adoption of ASU 2014-09 (Details) - USD ($) $ / shares in Units, $ 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
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	$ 742,365	$ 693,227	[1]	$ 1,439,557	$ 1,355,904	[1]
Total cost of net revenues	108,749	109,312		217,035	211,770
Gross margin	633,616	583,915		1,222,522	1,144,134
Total operating expenses	488,469	461,974		911,812	899,549
Income from operations	145,147	121,941		310,710	244,585
Net income	$ 106,833	$ 108,829		$ 251,092	$ 136,450
Basic earnings per share (in dollars per share)	$ 0.79	$ 0.72		$ 1.82	$ 0.90
Diluted earnings per share (in dollars per share)	$ 0.73	$ 0.70		$ 1.72	$ 0.87
Prepaid and other current assets	$ 208,648			$ 208,648				$ 147,330
Other assets	104,015			104,015				52,685
Deferred tax assets, net	119,760			119,760				152,362
Total assets	5,366,062			5,366,062				5,820,176
Other liabilities	143,677			143,677				121,936
Current portion of deferred revenues	1,243,032			1,243,032			$ 1,250,000	1,308,474
Long-term portion of deferred revenues	480,942			480,942			512,800	555,769
Total liabilities	4,701,805			4,701,805
Retained earnings	3,893,354			3,893,354				$ 3,509,484
Balances without adoption of ASC 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	741,920			1,449,244
Total cost of net revenues	107,807			215,933
Gross margin	634,113			1,233,311
Total operating expenses	499,047			925,731
Income from operations	135,066			307,580
Net income	$ 99,295			$ 250,197
Basic earnings per share (in dollars per share)	$ 0.73			$ 1.82
Diluted earnings per share (in dollars per share)	$ 0.68			$ 1.72
Prepaid and other current assets	$ 186,538			$ 186,538
Other assets	42,323			42,323
Deferred tax assets, net	150,444			150,444
Total assets	5,312,944			5,312,944
Other liabilities	130,617			130,617
Current portion of deferred revenues	1,290,041			1,290,041
Long-term portion of deferred revenues	525,160			525,160
Total liabilities	4,779,972			4,779,972
Retained earnings	3,762,069			3,762,069
Accounting Standards Update 2014-09 [Member] | Effect of Change Higher/(Lower) [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	445			(9,687)
Total cost of net revenues	942			1,102
Gross margin	(497)			(10,789)
Total operating expenses	(10,578)			(13,919)
Income from operations	10,081			3,130
Net income	$ 7,538			$ 895
Basic earnings per share (in dollars per share)	$ 0.06			$ 0.00
Diluted earnings per share (in dollars per share)	$ 0.05			$ 0.00
Prepaid and other current assets	$ 22,110			$ 22,110
Other assets	61,692			61,692
Deferred tax assets, net	(30,684)			(30,684)
Total assets	53,118			53,118
Other liabilities	13,060			13,060
Current portion of deferred revenues	(47,009)			(47,009)
Long-term portion of deferred revenues	(44,218)			(44,218)
Total liabilities	(78,167)			(78,167)
Retained earnings	131,285			131,285			130,700
Contract assets							$ 7,300
Prepaid Expenses and Other Current Assets [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract acquisition costs	35,000			35,000
Prepaid Expenses and Other Current Assets [Member] | Balances without adoption of ASC 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract acquisition costs	14,200			14,200
Other Noncurrent Assets [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract acquisition costs	59,500			59,500
Other Noncurrent Liabilities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax liabilities	62,800			62,800
Other Noncurrent Liabilities [Member] | Balances without adoption of ASC 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax liabilities	$ 49,700			$ 49,700

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