CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income	$ 433,887	$ 451,295	$ 419,375
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	85,275	60,516	59,255
Operating lease right-of-use assets amortization	20,971	18,459
Deferred income tax benefit	(30,932)	(14,511)	(33,675)
Provision for bad debts	6,438	2,928	1,577
Stock-based compensation expense	145,615	116,190	83,346
Other	2,180	2,778	410
Changes in operating assets and liabilities:
Accounts receivable	(160,319)	(154,403)	(74,455)
Other receivables and current assets	(2,312)	(26,182)	(30,241)
Other long-term assets	(14,818)	(5,622)	1,839
Accounts payable, accrued expenses and current liabilities	21,362	38,543	19,920
Accrued income taxes	19,713	575	1,086
Deferred revenue	5,448	17,245	56,213
Other long-term liabilities	14,802	(7,875)	(19,662)
Net cash provided by operating activities	547,310	499,936	484,988
Cash flows from investing activities:
Acquisitions, net of cash acquired	(572,328)	(787,196)	(283,026)
Capital expenditures	(35,370)	(44,940)	(21,762)
Other investing activities	(6,555)	(1,412)	(7,443)
Net cash used in investing activities	(614,253)	(833,548)	(312,231)
Cash flows from financing activities:
Proceeds from long-term debt	375,000	500,000	0
Principal payments on long-term debt	(75,000)	0	0
Purchase of treasury stock	(161,029)	(59,116)	(269,801)
Restricted stock withholding taxes paid in lieu of issued shares	(71,019)	(42,431)	(28,879)
Proceeds from shares issued for stock-based compensation	29,560	34,093	41,019
Other financing activities	(915)	(3,137)	(5,014)
Net cash provided by (used in) financing activities	96,597	429,409	(262,675)
Effect of exchange rate fluctuations on cash and cash equivalents	10,924	(842)	(14,444)
Net increase (decrease) in cash and cash equivalents	40,578	94,955	(104,362)
Cash and cash equivalents, beginning of period	872,094	777,139	881,501
Cash and cash equivalents, end of period	912,672	872,094	777,139
Supplemental disclosures of cash flow information:
Income taxes paid	54,174	86,770	87,244
Interest paid	11,941	787	114
Fair value of common stock issued and unpaid consideration in connection with acquisitions	$ 232,690	$ 307,173	$ 0