Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities
Net income (loss)	$ 114,833	$ (36,466)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities
Stock-based compensation expense	7,272	5,074
Net amortization of premiums and accretion of discounts on marketable debt securities	718	945
Depreciation and amortization	1,730	1,257
Net realized gains on sales of marketable securities		(480)
Imputed interest income from licensing	(8,253)
Other non-cash adjustments	13	40
Changes in operating assets and liabilities
Accounts receivable	8,879	982
Prepaid expenses	1,644	(657)
Other current assets	(585)	(242)
Other assets	(652)	(86)
Accounts payable	(340)	2,723
Accrued expenses and other current liabilities	2,566	(31)
Deferred revenue	600
Deferred rent	19	(89)
Other liabilities	(99)
Net cash provided by (used in) operating activities	128,345	(27,030)
Cash flows from investing activities
Purchases of marketable securities	(139,081)	(46,593)
Maturities of marketable securities	68,645	28,010
Sales of marketable securities		780
Purchases of property and equipment	(5,017)	(4,609)
Net cash used in investing activities	(75,453)	(22,412)
Cash flows from financing activities
Proceeds from exercise of stock options	6,999	329
Proceeds from issuance of common stock under employee stock purchase plan	342	147
Proceeds from public offering of common stock, net of underwriting discounts and commissions		81,994
Issuance costs for public offering of common stock		(219)
Net cash provided by financing activities	7,341	82,251
Net increase in cash and cash equivalents and restricted cash	60,233	32,809
Cash and cash equivalents and restricted cash
Beginning of period	46,881	25,065
End of period	$ 107,114	57,874
Supplemental disclosures of non-cash investing and financing activities
Issuance costs for public offering of common stock in accounts payable and accrued expenses		$ 193