CONSOLIDATED STATEMENTS OF CASH FLOWS $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Cash flows from operating activities:
Net income	$ 327,160	$ 261,057	$ 240,256
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization expense	62,874	45,317	36,640
Operating lease right-of-use assets amortization expense	66,369	55,859	0
Bad debt expense	2,253	1,619	848
Deferred taxes	(19,994)	(7,764)	(48,000)
Stock-based compensation expense	75,238	72,036	59,188
Other	6,796	4,764	(1,712)
Changes in assets and liabilities:
Trade receivables and contract assets	4,235	(87,174)	(46,902)
Prepaid and other assets	6,983	(7,155)	(8,432)
Accounts payable	2,428	(1,685)	(772)
Accrued expenses and other liabilities	60,133	27,125	51,953
Operating lease liabilities	(64,453)	(53,419)	0
Income taxes payable	14,385	(23,127)	9,151
Net cash provided by operating activities	544,407	287,453	292,218
Cash flows from investing activities:
Purchases of property and equipment	(68,793)	(99,308)	(37,574)
Payments to Acquire Other Investments	(120,000)	0	0
Proceeds from short-term investments	60,009	0	418
Acquisition of businesses, net of cash acquired (Note 2)	(18,888)	(39,322)	(74,268)
Purchases of non-marketable securities	(20,500)	(5,000)	0
Other investing activities, net	1,018	(1,739)	(699)
Net cash used in investing activities	(167,154)	(145,369)	(112,123)
Cash flows from financing activities:
Proceeds from stock option exercises	26,410	37,003	34,845
Payments of withholding taxes related to net share settlements of restricted stock units	(20,132)	(15,503)	(7,747)
Repayment of debt (Note 9)	(18)	(9)	(3,494)
Payment of contingent consideration for previously acquired businesses	(7,004)	(1,104)	0
Other financing activities, net	(21)	(24)	(603)
Net cash (used in)/provided by financing activities	(765)	20,363	23,001
Effect of exchange rate changes on cash, cash equivalents and restricted cash	9,357	3,530	(14,240)
Net increase in cash, cash equivalents and restricted cash	385,845	165,977	188,856
Cash, cash equivalents and restricted cash, beginning of period	937,688	771,711	582,855
Cash, cash equivalents and restricted cash, end of period	1,323,533	937,688	771,711
Cash paid during the year for:
Income taxes, net of refunds	54,520	65,306	40,437
Interest	425	832	777
Supplemental disclosure of non-cash investing and financing activities
Acquisition-date fair value of contingent consideration issued for acquisition of businesses	7,119	3,876	8,390
Capital expenditures incurred but not yet paid	$ 1,582	$ 16,921	$ 2,140