Summary of Significant Accounting Policies (Newly-Adopted Accounting Pronouncements) (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, 2019	Jan. 01, 2018	Dec. 31, 2015
Current assets:
Accounts receivable	$ 479,889				$ 461,457				$ 479,889	$ 461,457
Prepaid expenses and other current assets	163,360				172,853				163,360	172,853
Total current assets	2,535,354				1,346,246				2,535,354	1,346,246
Deferred income tax assets	34,913				36,231				34,913	36,231
Other assets	116,067				136,365				116,067	136,365
Total assets	5,461,770				4,648,916				5,461,770	4,648,916
Current liabilities:
Deferred revenue	69,083				70,495				69,083	70,495
Total current liabilities	1,210,709				456,694				1,210,709	456,694
Deferred revenue	4,557				6,062				4,557	6,062
Deferred income tax liabilities	19,624				17,823				19,624	17,823
Total liabilities	2,269,910				1,286,447				2,269,910	1,286,447
Stockholders’ equity:
Accumulated deficit	(430,890)				(690,662)				(430,890)	(690,662)
Total stockholders’ equity	3,191,860				3,362,469				3,191,860	3,362,469	$ 3,270,218			$ 3,162,865
Total liabilities and stockholders’ equity	5,461,770				4,648,916				5,461,770	4,648,916
Revenue	713,363	$ 669,628	$ 662,759	$ 668,724	658,470	$ 624,440	$ 605,832	$ 600,293	2,714,474	2,489,035	2,347,988
Costs and operating expenses:
Cost of revenue (exclusive of amortization of acquired intangible assets shown below)	243,927	239,246	235,487	234,825	229,940	225,490	214,680	205,727	953,485	875,837	809,106
Sales and marketing									517,353	481,522	427,885
Total costs and operating expenses									2,351,975	2,174,746	1,881,478
Income from operations									362,499	314,289	466,510
Income before provision for income taxes									343,089	314,192	466,362
Provision for income taxes									44,716	91,426	145,635
Net income	$ 94,015	$ 107,583	$ 43,061	$ 53,714	$ 27,520	$ 63,911	$ 56,752	$ 74,583	$ 298,373	$ 222,766	$ 320,727
Net income per share:
Basic (in dollars per share)	$ 0.58	$ 0.65	$ 0.25	$ 0.32	$ 0.16	$ 0.37	$ 0.33	$ 0.43	$ 1.78	$ 1.30	$ 1.83
Diluted (in dollars per share)	$ 0.57	$ 0.64	$ 0.25	$ 0.31	$ 0.16	$ 0.37	$ 0.33	$ 0.43	$ 1.76	$ 1.29	$ 1.82
Cash flows from operating activities:
Net income	$ 94,015	$ 107,583	$ 43,061	$ 53,714	$ 27,520	$ 63,911	$ 56,752	$ 74,583	$ 298,373	$ 222,766	$ 320,727
Adjustments to reconcile net income to net cash provided by operating activities:
Provision (benefit) for deferred income taxes									2,339	(7,244)	9,629
Changes in operating assets and liabilities, net of effects of acquisitions and divestitures:
Accounts receivable									(30,445)	(50,054)	(2,970)
Prepaid expenses and other current assets									(4,132)	(28,968)	42,145
Deferred revenue									(919)	2,938	(3,512)
Other non-current assets and liabilities									14,142	14,442	(26,202)
Net cash provided by operating activities									1,008,327	800,983	871,812
Cash flows from investing activities:
Other non-current assets and liabilities									(2,066)	(1,586)	786
Net cash used in by investing activities									(506,533)	(459,165)	(469,426)
Effects of exchange rate changes on cash and cash equivalents									(12,844)	12,990	(7,891)
Net increase (decrease) in cash and cash equivalents									722,558	(10,197)	34,716
Cash, cash equivalents and restricted cash at beginning of year				314,429				324,626	314,429	324,626	289,910
Cash, cash equivalents and restricted cash at end of year	$ 1,036,987				314,429				1,036,987	314,429	324,626
Intra-Entity Asset Transfers | Deferred costs included in prepaid and other current assets
Net income per share:
Cumulative effect adjustment on assets													$ 11,600
Intra-Entity Asset Transfers | Deferred costs included in other assets
Net income per share:
Cumulative effect adjustment on assets													$ 27,000
As Previously Reported
Current assets:
Accounts receivable					459,127					459,127
Prepaid expenses and other current assets					137,809					137,809
Total current assets					1,308,872					1,308,872
Deferred income tax assets					51,069					51,069
Other assets					112,829					112,829
Total assets					4,602,844					4,602,844
Current liabilities:
Deferred revenue					77,705					77,705
Total current liabilities					463,904					463,904
Deferred revenue					6,839					6,839
Deferred income tax liabilities					15,510					15,510
Total liabilities					1,292,121					1,292,121
Stockholders’ equity:
Accumulated deficit					(742,408)					(742,408)
Total stockholders’ equity					3,310,723					3,310,723
Total liabilities and stockholders’ equity					4,602,844					4,602,844
Revenue										2,502,996	2,340,049
Costs and operating expenses:
Cost of revenue (exclusive of amortization of acquired intangible assets shown below)										875,758	809,001
Sales and marketing										493,632	426,967
Total costs and operating expenses										2,186,777	1,880,455
Income from operations										316,219	459,594
Income before provision for income taxes										316,122	459,446
Provision for income taxes										97,801	143,314
Net income										$ 218,321	$ 316,132
Net income per share:
Basic (in dollars per share)										$ 1.27	$ 1.81
Diluted (in dollars per share)										$ 1.26	$ 1.79
Cash flows from operating activities:
Net income										$ 218,321	$ 316,132
Adjustments to reconcile net income to net cash provided by operating activities:
Provision (benefit) for deferred income taxes										(869)	7,308
Changes in operating assets and liabilities, net of effects of acquisitions and divestitures:
Accounts receivable										(63,825)	3,356
Prepaid expenses and other current assets										(22,311)	23,881
Deferred revenue										1,142	(1,213)
Other non-current assets and liabilities										21,422	(9,647)
Net cash provided by operating activities										800,983	871,812
Cash flows from investing activities:
Other non-current assets and liabilities										(2,098)	782
Net cash used in by investing activities										(459,677)	(469,430)
Effects of exchange rate changes on cash and cash equivalents										12,912	(7,907)
Net increase (decrease) in cash and cash equivalents										(10,787)	34,696
Cash, cash equivalents and restricted cash at beginning of year				313,382				324,169	313,382	324,169	289,473
Cash, cash equivalents and restricted cash at end of year					313,382					313,382	324,169
Standard Adjustments | Revenue Recognition Standard Adjustments
Current assets:
Accounts receivable					2,330					2,330
Prepaid expenses and other current assets					35,044					35,044
Total current assets					37,374					37,374
Deferred income tax assets					(14,838)					(14,838)
Other assets					23,536					23,536
Total assets					46,072					46,072
Current liabilities:
Deferred revenue					(7,210)					(7,210)
Total current liabilities					(7,210)					(7,210)
Deferred revenue					(777)					(777)
Deferred income tax liabilities					2,313					2,313
Total liabilities					(5,674)					(5,674)
Stockholders’ equity:
Accumulated deficit					51,746					51,746
Total stockholders’ equity					51,746					51,746
Total liabilities and stockholders’ equity					46,072					46,072
Revenue										(13,961)	7,939
Costs and operating expenses:
Cost of revenue (exclusive of amortization of acquired intangible assets shown below)										79	105
Sales and marketing										(12,110)	918
Total costs and operating expenses										(12,031)	1,023
Income from operations										(1,930)	6,916
Income before provision for income taxes										(1,930)	6,916
Provision for income taxes										(6,375)	2,321
Net income										$ 4,445	$ 4,595
Net income per share:
Basic (in dollars per share)										$ 0.03	$ 0.02
Diluted (in dollars per share)										$ 0.03	$ 0.03
Adjustment to foreign currency translation adjustment										$ 1,500	$ (700)
Adjustment to net income										4,400	4,600
Cash flows from operating activities:
Net income										4,445	4,595
Adjustments to reconcile net income to net cash provided by operating activities:
Provision (benefit) for deferred income taxes										(6,375)	2,321
Changes in operating assets and liabilities, net of effects of acquisitions and divestitures:
Accounts receivable										13,771	(6,326)
Prepaid expenses and other current assets										(6,657)	18,264
Deferred revenue										1,796	(2,299)
Other non-current assets and liabilities										(6,980)	(16,555)
Net cash provided by operating activities										0	0
Cash flows from investing activities:
Other non-current assets and liabilities										0	0
Net cash used in by investing activities										0	0
Effects of exchange rate changes on cash and cash equivalents										0	0
Net increase (decrease) in cash and cash equivalents										0	0
Cash, cash equivalents and restricted cash at beginning of year				0				0	0	0	0
Cash, cash equivalents and restricted cash at end of year					0					0	0
Standard Adjustments | Cash Flow Standard Adjustments
Costs and operating expenses:
Net income										0	0
Cash flows from operating activities:
Net income										0	0
Adjustments to reconcile net income to net cash provided by operating activities:
Provision (benefit) for deferred income taxes										0	0
Changes in operating assets and liabilities, net of effects of acquisitions and divestitures:
Accounts receivable										0	0
Prepaid expenses and other current assets										0	0
Deferred revenue										0	0
Other non-current assets and liabilities										0	0
Net cash provided by operating activities										0	0
Cash flows from investing activities:
Other non-current assets and liabilities										512	4
Net cash used in by investing activities										512	4
Effects of exchange rate changes on cash and cash equivalents										78	16
Net increase (decrease) in cash and cash equivalents										590	20
Cash, cash equivalents and restricted cash at beginning of year				$ 1,047				$ 457	$ 1,047	457	437
Cash, cash equivalents and restricted cash at end of year					$ 1,047					$ 1,047	$ 457
Subsequent Event | Lease ASU
Cash flows from investing activities:
Right-of-use lease assets												$ 350,000
Lease liabilities												$ 375,000