CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows provided by (used in) operating activities:
Net income (loss)	$ 62,125	$ (27,710)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization of property and equipment	439	469
Stock-based compensation	3,752	2,735
Deferred income taxes	66	(77)
Allowance for doubtful accounts	0	6
Loss on disposal of equipment	26	0
Changes in operating assets and liabilities:
Accounts and other receivables	(220)	(1,959)
Prepaid expenses and other current assets	(215)	(112)
Other assets	(4,077)	(99)
Accounts payable	(3,288)	4,286
Accrued compensation and other current liabilities	(656)	(2,902)
Income tax payable	(147)	0
Deferred revenue	23,917	(2,929)
Other long-term liabilities	239	(50)
Net cash provided by (used in) operating activities	81,961	(28,342)
Cash flows provided by investing activities:
Purchases of short-term investments	(17,693)	(15,879)
Proceeds from maturities of short-term investments	19,500	20,000
Purchases of property and equipment	(31)	(110)
Net cash provided by investing activities	1,776	4,011
Cash flows provided by financing activities:
Issuance of common stock under employee stock purchase plan	98	175
Exercise of stock options	7,980	443
Net cash provided by financing activities	8,078	618
Net increase (decrease) in cash and cash equivalents	91,815	(23,713)
Cash and cash equivalents:
Beginning of period	24,622	56,865
End of period	116,437	33,152
Supplemental disclosure of cash flow information
Cash paid for taxes	81	111
Supplemental disclosure of noncash operating, investing, and financing activities
Amounts accrued for property and equipment	0	3
Amounts accrued for treasury stocks	569	0
Release of Restricted Stock Units and Awards under company stock plan	$ 2,546	$ 2,451