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	$ 48,888	$ 61,733	$ 51,565
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	38,984	34,580	31,465
Stock-based compensation expense	23,041	19,770	16,047
Unrealized foreign exchange (gain)/loss	1,523	(1,001)	(3,798)
Deferred income tax (benefit)/expense	731	(3,384)	2,238
Excess tax benefit from stock-based compensation	0	(4,326)	(1,458)
Change in fair value of earn-out consideration	0	(4,060)	0
Allowance for doubtful accounts receivable	2,816	0	0
Others, net	252	(107)	(278)
Change in operating assets and liabilities (net of effect of acquisitions):
Restricted cash	(19)	(2,137)	(787)
Accounts receivable	(20,482)	(18,062)	(9,087)
Prepaid expenses and other current assets	218	(5,421)	(3,112)
Accounts payable	1,706	(2,628)	44
Deferred revenue	(6,625)	5,726	2,566
Accrued employee costs	6,391	5,304	8,528
Accrued expenses and other liabilities	6,903	9,080	(4,699)
Advance income tax, net	11,037	437	8,865
Other assets	(2,224)	4,754	(1,408)
Net cash provided by operating activities	113,140	100,258	96,691
Cash flows from investing activities:
Purchase of property and equipment	(35,154)	(25,850)	(25,585)
Investment in equity affiliate	(3,000)	0	0
Business acquisitions (net of cash acquired)	(23,300)	(28,666)	(44,270)
Purchase of investments	(402,721)	(182,471)	(129,050)
Proceeds from redemption of investments	241,439	182,320	125,365
Net cash used for investing activities	(222,736)	(54,667)	(73,540)
Cash flows from financing activities:
Principal payments on capital lease obligations	(174)	(348)	(720)
Proceeds from borrowings	60,574	0	30,000
Repayments of borrowings	(45,192)	(25,000)	(10,000)
Proceeds from non-controlling interest	0	0	176
Payment of debt issuance costs	(790)	0	(74)
Acquisition of treasury stock	(43,454)	(18,203)	(14,195)
Proceeds from exercise of stock options	8,561	6,499	3,375
Excess tax benefit from stock-based compensation	0	4,326	1,458
Net cash (used for)/provided by financing activities	(20,475)	(32,726)	10,020
Effect of exchange rate changes on cash and cash equivalents	3,711	(5,033)	(4,347)
Net increase/(decrease) in cash and cash equivalents	(126,360)	7,832	28,824
Cash and cash equivalents, beginning of year	213,155	205,323	176,499
Cash and cash equivalents, end of year	86,795	213,155	205,323
Supplemental disclosure of cash flow information:
Cash paid for interest	1,122	1,178	1,188
Cash paid for taxes, net of refund	19,128	15,667	11,505
Assets acquired under capital lease	$ 301	$ 334	$ 215