CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows provided by (used in) operating activities:
Net income (loss)	$ 69,883	$ (12,865)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization of property and equipment	227	232
Stock-based compensation	1,222	1,557
Deferred income taxes	(67)	0
Loss on disposal of equipment	26	0
Changes in operating assets and liabilities:
Accounts and other receivables	(1,861)	(527)
Prepaid expenses and other current assets	116	150
Other assets	(4,524)	(55)
Accounts payable	(2,391)	1,000
Accrued compensation and other current liabilities	(2,181)	(1,741)
Deferred revenue	25,197	(2,046)
Other long-term liabilities	422	(22)
Net cash provided by (used in) operating activities	86,203	(14,317)
Cash flows provided by (used in) investing activities:
Purchases of short-term investments	(8,861)	(9,931)
Proceeds from maturities of short-term investments	11,000	10,000
Purchases of property and equipment	(6)	(83)
Net cash provided by (used in) investing activities	2,133	(14)
Cash flows provided by financing activities:
Issuance of common stock under employee stock purchase plan	98	175
Exercise of stock options	6,072	401
Net cash provided by financing activities	6,170	576
Net increase (decrease) in cash and cash equivalents	94,506	(13,755)
Cash and cash equivalents:
Beginning of period	24,622	56,865
End of period	119,128	43,110
Supplemental disclosure of cash flow information
Cash paid for taxes	90	39
Supplemental disclosure of noncash operating, investing, and financing activities
Amounts accrued for property and equipment	0	11
Release of Restricted Stock Units and Awards under company stock plan	$ 1,860	$ 1,768