CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OPERATING ACTIVITIES:
Net income (loss)	$ 55,350	$ 152,995	$ (1,711)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	42,807	41,198	35,346
Provision for doubtful accounts	24,515	20,917	18,907
Stock-based compensation	114,386	100,415	86,261
Deferred income taxes	(15,469)	0	1,351
Gain on disposal of a business unit	0	(163,697)	0
Other adjustments	(722)	1,512	2,973
Changes in operating assets and liabilities net of acquisitions and disposals of a business unit:
Accounts receivable	(35,664)	(36,146)	(34,618)
Prepaid expenses and other assets	(5,192)	(2,581)	2,728
Accounts payable, accrued expenses and other liabilities	(20,204)	52,882	15,278
Deferred revenue	380	152	385
Net cash provided by operating activities	160,187	167,647	126,900
INVESTING ACTIVITIES:
Purchases of marketable securities	(751,237)	(354,895)	(274,965)
Maturities of marketable securities	613,700	264,000	265,500
Sale of investment prior to maturity	17,895	0	0
Purchase of cost-method investment	0	0	(8,000)
Sale of a business, net of cash sold	0	252,663	0
Acquisitions, net of cash received	0	(50,544)	0
Purchases of property, equipment and software	(24,849)	(15,598)	(22,994)
Capitalized website and software development costs	(20,123)	(14,647)	(14,191)
Other investing activities	245	157	(91)
Net cash (used in)/provided by investing activities	(164,369)	81,136	(54,741)
FINANCING ACTIVITIES:
Proceeds from issuance of common stock for employee stock-based plans	29,779	40,917	29,522
Taxes paid related to net share settlement of equity awards	(50,144)	(1,199)	0
Repurchases of common stock	(187,382)	(12,556)	0
Net cash (used in) provided by financing activities	(207,747)	27,162	29,522
Effect of exchange rate changes on cash, cash equivalents and restricted cash	360	941	(262)
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(211,569)	276,886	101,419
CASH, CASH EQUIVALENTS AND RESTRICTED CASH—Beginning of period	566,404	289,518	188,099
CASH, CASH EQUIVALENTS AND RESTRICTED CASH—End of period	354,835	566,404	289,518
SUPPLEMENTAL DISCLOSURES OF OTHER CASH FLOW INFORMATION:
Cash paid for income taxes, net of refunds	29,159	530	813
SUPPLEMENTAL DISCLOSURES OF NON CASH INVESTING AND FINANCING ACTIVITIES:
Purchases of property, equipment and software recorded in accounts payable, accrued liabilities and long-term liabilities	4,440	11,493	989
Goodwill measurement period adjustment	0	(178)	146
Tax liability related to net share settlement of equity awards included in accrued liabilities	$ 971	$ 1,323	$ 0