Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 218,321	$ 316,132	$ 321,406
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	372,313	334,302	299,563
Stock-based compensation	164,308	144,506	126,677
(Benefit) provision for deferred income taxes	(869)	7,308	4,098
Amortization of debt discount and issuance costs	18,839	18,638	18,525
Restructuring-related software charges	31,965	4,587	0
Other non-cash reconciling items, net	10,068	5,987	5,804
Changes in operating assets and liabilities, net of effects of acquisitions and divestitures:
Accounts receivable	(63,825)	3,356	(56,247)
Prepaid expenses and other current assets	(22,311)	23,881	7,137
Accounts payable and accrued expenses	33,232	18,491	51,624
Deferred revenue	1,142	(1,213)	3,224
Other current liabilities	16,378	5,484	(345)
Other non-current assets and liabilities	21,422	(9,647)	11,986
Net cash provided by operating activities	800,983	871,812	793,452
Cash flows from investing activities:
Cash paid for acquisitions, net of cash acquired	(369,073)	(95,439)	(141,147)
Purchases of property and equipment	(254,146)	(180,949)	(311,676)
Capitalization of internal-use software development costs	(160,632)	(135,340)	(133,307)
Purchases of short- and long-term marketable securities	(326,497)	(781,061)	(692,879)
Proceeds from sales of short and long-term marketable securities	219,916	57,740	2,008
Proceeds from maturities of short and long-term marketable securities	432,853	664,837	843,931
Other non-current assets and liabilities	(2,098)	782	(2,494)
Net cash used in by investing activities	(459,677)	(469,430)	(435,564)
Cash flows from financing activities:
Proceeds related to the issuance of common stock under stock plans	55,680	59,560	61,791
Employee taxes paid related to net share settlement of stock-based awards	(58,395)	(45,545)	(54,164)
Repurchases of common stock	(361,194)	(373,794)	(302,606)
Other non-current assets and liabilities	(1,096)	0	(2,050)
Net cash used in financing activities	(365,005)	(359,779)	(297,029)
Effects of exchange rate changes on cash and cash equivalents	12,912	(7,907)	(10,036)
Net (decrease) increase in cash and cash equivalents	(10,787)	34,696	50,823
Cash and cash equivalents at beginning of year	324,169	289,473	238,650
Cash and cash equivalents at end of year	313,382	324,169	289,473
Supplemental disclosure of cash flow information:
Cash paid for income taxes, net of refunds received in the years ended December 31, 2017, 2016 and 2015 of $6,750, $1,664 and $19,374, respectively	91,640	120,223	75,033
Non-cash financing and investing activities:
Purchases of property and equipment and capitalization of internal-use software development costs included in accounts payable and accrued expenses	27,209	36,742	19,327
Capitalization of stock-based compensation	$ 28,851	$ 23,093	$ 17,867