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 (loss)	$ 22,869	$ (70,279)	$ (48,882)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	10,857	7,488	4,551
Stock-based compensation	46,670	62,875	50,964
Amortization of acquired intangibles	714	1,408	1,408
Amortization of premiums/accretion of discounts on short-term investments	(25)	238	496
Loss on disposal of equipment	74	267	0
Excess tax benefits from share-based compensation	0	(36)	(165)
Gain on sale of business	(35,871)	0	0
Other	(1,299)	125	0
Changes in operating assets and liabilities:
Accounts receivable, net	(12,964)	(1,520)	(13,605)
Inventory	(93)	(16)	(1,056)
Prepaid expenses and other assets	(1,959)	383	(4,220)
Accounts payable	115	(1,398)	977
Accrued compensation and benefits	5,197	(2,543)	4,510
Accrued and other liabilities	3,378	2,178	3,242
Severance pay, net	58	272	16
Deferred revenue	30,237	23,684	25,482
Deferred tax assets	(802)	(632)	149
Net cash provided by operating activities	67,156	22,494	23,867
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of short-term investments	(130,972)	(129,989)	(89,626)
Proceeds from sales/maturities of short-term investments	117,552	72,453	33,948
Proceeds from sale of business	35,015	0	0
Acquisitions, net of cash acquired	0	(3,914)	0
Net purchases of property and equipment	(13,924)	(16,789)	(8,080)
Change in restricted cash	(384)	(208)	(17)
Net cash provided by (used in) investing activities	7,287	(78,447)	(63,775)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net of repurchases	19,095	11,233	23,524
Shares withheld for tax withholding on vesting of restricted stock units	(9,642)	(8,831)	(11,464)
Offering costs relating to follow-on public offering	0	(112)	0
Proceeds from follow-on public offering, net of offering costs	0	0	127,853
Settlement of holdback liability	0	(7,157)	0
Excess tax benefits from share-based compensation	0	36	165
Net cash provided by (used in) financing activities	9,453	(4,831)	140,078
Effect of exchange rate changes on cash and cash equivalents	1,299	(125)	(14)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	85,195	(60,909)	100,156
CASH AND CASH EQUIVALENTS - Beginning of period	107,343	168,252	68,096
CASH AND CASH EQUIVALENTS - End of period	192,538	107,343	168,252
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for income taxes	1,409	1,480	652
NONCASH INVESTING AND FINANCING ACTIVITIES:
Property and equipment incurred but not yet paid	1,645	574	517
Vesting of restricted and early exercised stock options	$ 0	$ 0	$ 767