Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities
Net income (loss)	$ (32,228)	$ 104,239
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities
Stock-based compensation expense	5,718	3,291
Net amortization of premiums and accretion of discounts on marketable debt securities	(368)	378
Depreciation and amortization	1,614	834
Imputed interest income from licensing	(613)	(1,355)
Other non-cash adjustments	327	10
Changes in operating assets and liabilities
Accounts receivable	591	(51,414)
Prepaid expenses	(790)	667
Other current assets	(295)	(446)
Operating lease right-of-use assets	573
Other assets	26	(21)
Accounts payable	316	477
Accrued expenses and other current liabilities	(2,932)	490
Operating lease liabilities	(592)
Deferred rent		27
Other liabilities	(644)	957
Net cash provided by (used in) operating activities	(29,297)	58,134
Cash flows from investing activities
Purchases of marketable securities	(79,249)	(54,267)
Maturities of marketable securities	87,165	19,525
Purchases of property and equipment	(2,455)	(2,344)
Net cash provided by (used in) investing activities	5,461	(37,086)
Cash flows from financing activities
Proceeds from exercise of stock options	3,762	3,824
Proceeds from issuance of common stock under employee stock purchase plan	365	342
Net cash provided by financing activities	4,127	4,166
Net increase (decrease) in cash and cash equivalents and restricted cash	(19,709)	25,214
Cash and cash equivalents and restricted cash
Beginning of period	76,614	46,881
End of period	$ 56,905	$ 72,095