CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows provided by (used in) operating activities:
Net income (loss)	$ (11,016)	$ 69,883
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization	205	227
Stock-based compensation	2,103	1,222
Deferred income taxes	117	67
Other non cash items	0	26
Changes in operating assets and liabilities:
Accounts and other receivables	(454)	(1,861)
Prepaid expenses and other current assets	1,028	116
Other assets	(11,419)	(4,524)
Accounts payable	8,763	(2,391)
Accrued compensation	(2,588)	(1,581)
Other current liabilities	2,092	(600)
Deferred revenue	(904)	25,197
Other long-term liabilities	4,495	422
Net cash provided by (used in) operating activities	(7,578)	86,203
Cash flows provided by (used in) investing activities:
Purchases of short-term investments	(8,930)	(8,861)
Proceeds from maturities of short-term investments	8,000	11,000
Purchases of property and equipment	(6)	(6)
Net cash provided by (used in) investing activities	(936)	2,133
Cash flows provided by financing activities:
Proceeds from issuance of common stock under employee stock purchase plan	109	98
Proceeds from stock options exercises	71	6,072
Net cash provided by financing activities	180	6,170
Net increase (decrease) in cash and cash equivalents	(8,334)	94,506
Cash and cash equivalents:
Beginning of period	110,988	24,622
End of period	102,654	119,128
Supplemental disclosure of cash flow information:
Cash paid for taxes	23	90
Supplemental disclosure of noncash operating, investing, and financing activities:
Release of restricted stock units and awards under company stock plan	$ 6,680	$ 1,860