CONSOLIDATED STATEMENTS OF CASH FLOWS $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Cash flows from operating activities:
Net income	$ 240,256	$ 72,760	$ 99,266
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization expense	36,640	28,562	23,387
Bad debt expense	848	51	1,539
Deferred taxes	(48,000)	12,561	(3,304)
Stock-based compensation expense	59,188	52,407	49,244
Excess tax benefit on stock-based compensation plans	0	0	(5,264)
Other	(1,712)	(4,010)	6,228
(Increase)/decrease in operating assets:
Accounts receivable	(31,005)	(58,745)	(30,612)
Unbilled revenues	(15,897)	(22,743)	34,777
Prepaid expenses and other assets	(8,432)	1,061	(2,391)
Increase/(decrease) in operating liabilities:
Accounts payable	(772)	1,221	741
Accrued expenses and other liabilities	41,929	37,282	(13,926)
Due to employees	1,535	1,933	5,261
Taxes payable	17,640	70,480	2,271
Net cash provided by operating activities	292,218	192,820	167,217
Cash flows from investing activities:
Purchases of property and equipment	(37,574)	(29,806)	(29,317)
Decrease in time deposits, net	418	0	29,597
Acquisition of businesses, net of cash acquired (Note 2)	(74,268)	(6,810)	(5,500)
Other investing activities, net	(699)	465	(4,100)
Net cash used in investing activities	(112,123)	(36,151)	(9,320)
Cash flows from financing activities:
Proceeds from stock option exercises	34,845	53,984	17,996
Excess tax benefit on stock-based compensation plans	0	0	5,264
Payments of withholding taxes related to net share settlements of restricted stock units	(7,747)	(3,194)	(539)
Proceeds from debt (Note 8)	0	25,000	20,000
Repayment of debt (Note 8)	(3,494)	(25,103)	(30,129)
Acquisition of businesses, deferred consideration (Note 2)	0	0	(2,260)
Other financing activities, net	(603)	(941)	135
Net cash provided by financing activities	23,001	49,746	10,467
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(14,240)	11,776	(3,387)
Net increase in cash, cash equivalents and restricted cash	188,856	218,191	164,977
Cash, cash equivalents and restricted cash, beginning of period	582,855	364,664	199,687
Cash, cash equivalents and restricted cash, end of period	771,711	582,855	364,664
Supplemental disclosures of cash flow information:
Income taxes	40,437	26,669	37,488
Bank interest	777	548	566
Supplemental disclosure of non-cash investing and financing activities
Acquisition-date fair value of contingent consideration issued for acquisition of businesses	$ 8,390	$ 0	$ 0