Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities
Net income (loss)	$ (33,685)	$ 114,833
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities
Stock-based compensation expense	12,817	7,272
Net amortization of premiums and accretion of discounts on marketable debt securities	(788)	718
Depreciation and amortization	3,363	1,730
Imputed interest income from licensing	(1,375)	(8,253)
Unrealized gains on marketable equity securities	31,656
Other non-cash adjustments	442	13
Changes in operating assets and liabilities
Accounts receivable	(1,419)	8,879
Prepaid expenses	(534)	1,644
Other current assets	1,419	(585)
Operating lease right-of-use assets	1,156
Other assets	(1,350)	(652)
Accounts payable	1,774	(340)
Accrued expenses and other current liabilities	(957)	2,566
Deferred revenue	(600)	600
Operating lease liabilities	(1,171)
Deferred rent		19
Other liabilities	(482)	(99)
Net cash provided by (used in) operating activities	(53,046)	128,345
Cash flows from investing activities
Purchases of marketable securities	(106,119)	(139,081)
Maturities of marketable securities	141,262	68,645
Purchases of property and equipment	(8,010)	(5,017)
Net cash provided by (used in) investing activities	27,133	(75,453)
Cash flows from financing activities
Proceeds from exercise of stock options	5,129	6,999
Proceeds from issuance of common stock under employee stock purchase plan	365	342
Net cash provided by financing activities	5,494	7,341
Net increase (decrease) in cash and cash equivalents and restricted cash	(20,419)	60,233
Cash and cash equivalents and restricted cash
Beginning of period	76,614	46,881
End of period	$ 56,195	$ 107,114