CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OPERATING ACTIVITIES:
Net income (loss)	$ 152,858	$ (4,670)	$ (32,900)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	41,198	35,346	29,604
Provision for doubtful accounts and sales returns	18,414	17,261	16,788
Stock-based compensation	100,415	86,261	60,842
Recording of valuation allowance	0	1,351	20,341
Excess tax benefit from stock-based award activity	0	0	(6,583)
Gain on disposal of a business unit	(164,779)	0	0
Other adjustments	(19)	1,625	1,399
Changes in operating assets and liabilities:
Accounts receivable	(32,112)	(31,624)	(25,279)
Prepaid expenses and other assets	(1,362)	5,687	(22,703)
Accounts payable, accrued expenses and other liabilities	52,882	15,278	15,894
Deferred revenue	152	385	(41)
Net cash provided by operating activities	167,647	126,900	57,362
INVESTING ACTIVITIES:
Purchases of marketable securities	(354,895)	(274,965)	(246,160)
Maturities of marketable securities	264,000	265,500	202,870
Purchase of cost-method investment	0	(8,000)	0
Sale of a business, net of cash sold	252,663	0	0
Acquisitions, net of cash received	(50,544)	0	(73,422)
Purchases of property, equipment and software	(15,598)	(22,994)	(31,127)
Capitalized website and software development costs	(14,647)	(14,191)	(11,734)
Other adjustments	(1,080)	(922)	891
Net cash provided by (used in) investing activities	79,899	(55,572)	(158,682)
FINANCING ACTIVITIES:
Proceeds from issuance of common stock for employee stock-based plans	40,917	29,522	21,166
Excess tax benefit from share-based award activity	0	0	6,583
Taxes paid related to net share settlement of equity awards	(1,199)	0	0
Repurchases of common stock	(12,556)	0	(482)
Contingent consideration payment	0	0	(825)
Net cash provided by financing activities	27,162	29,522	26,442
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	941	(262)	(821)
CHANGE IN CASH AND CASH EQUIVALENTS	275,649	100,588	(75,699)
CASH AND CASH EQUIVALENTS—Beginning of period	272,201	171,613	247,312
CASH AND CASH EQUIVALENTS—End of period	547,850	272,201	171,613
SUPPLEMENTAL DISCLOSURES OF OTHER CASH FLOW INFORMATION:
Cash paid for income taxes, net of refunds	530	813	352
SUPPLEMENTAL DISCLOSURES OF NONCASH INVESTING AND FINANCING ACTIVITIES:
Purchases of property, equipment and software recorded in accounts payable, accrued liabilities and long-term liabilities	11,493	989	2,233
Goodwill measurement period adjustment	(178)	146	(255)
Tax liability related to net share settlement of equity awards included in accrued liabilities	(1,323)	0	0
Issuance of common stock in connection with acquisition	$ 0	$ 0	$ 59,158