Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities
Net loss	$ (57,641)	$ (54,074)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	416	329
Stock-based compensation expense	8,708	8,244
In process research and development expenses	30	2,805
Amortization of operating lease right-of-use assets	858	566
Amortization of debt discount and issuance costs	1,605	89
Amortization of premium on investments, net of accretion of discounts	66	(65)
Net realized gain on investments	0	(39)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,855)	(1,785)
Other assets	16	925
Operating lease liabilities	117	(566)
Accounts payable	(6,565)	1,609
Accrued expenses	1,771	(2,239)
Accrued research and development expenses	1,078	(2,461)
Accrued compensation and benefits	(6,500)	(5,633)
Net cash used in operating activities	(58,896)	(52,295)
Cash flows from investing activities
Research and development asset acquisitions, net of cash acquired	(30)	(2,805)
Purchase of marketable securities	0	(73,778)
Maturities of marketable securities	12,800	89,083
Sales of marketable securities	0	16,051
Purchase of property and equipment	(802)	(513)
Net cash provided by investing activities	11,968	28,038
Cash flows from financing activities
Proceeds from the exercise of stock options	15	15
Net cash provided by financing activities	774	571
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(242)	(50)
Net decrease in cash, cash equivalents and restricted cash	(46,154)	(23,686)
Cash, cash equivalents and restricted cash, at the beginning of the period	486,620	135,089
Cash, cash equivalents and restricted cash, at the end of the period	440,224	111,353
Supplemental disclosure of cash flow information:
Cash paid for interest	675	618
Supplemental disclosure of noncash investing and financing activities:
Change in unrealized gain on marketable securities, net of tax	(69)	(676)
Unpaid property and equipment	0	60
Employee Stock Purchase Plan
Cash flows from financing activities
Proceeds from issuance of common stock	$ 759	$ 556