Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Cash flows from operating activities
Net loss	$ (117,473)	$ (120,944)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	842	679
Stock-based compensation expense	16,762	17,144
In process research and development expenses	45	17,805
Amortization of operating lease right-of-use assets	1,729	1,203
Amortization of debt discount and issuance costs	3,284	823
Amortization of premium on investments, net of accretion of discounts	115	(33)
Net realized gain on investments	0	(253)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,373)	(548)
Other assets	(43)	932
Operating lease liabilities	(1,778)	(715)
Accounts payable	(6,631)	(727)
Accrued expenses	(342)	(982)
Accrued research and development expenses	2,775	(6,103)
Accrued compensation and benefits	(4,258)	(2,975)
Net cash used in operating activities	(106,346)	(94,694)
Cash flows from investing activities
Research and development asset acquisitions, net of cash acquired	(45)	(17,805)
Purchase of marketable securities	(49,923)	(73,777)
Maturities of marketable securities	12,800	143,304
Sales of marketable securities	0	83,515
Purchase of property and equipment	(940)	(954)
Net cash provided by (used in) investing activities	(38,108)	134,283
Cash flows from financing activities
Proceeds from issuance of common stock in a public offering, net	0	117,094
Proceeds from issuance of convertible debt, net	0	193,596
Purchase of shares pursuant to Employee Stock Purchase Plan	759	556
Proceeds from the exercise of stock options	286	154
Net cash provided by financing activities	1,045	311,400
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(192)	(16)
Net increase (decrease) in cash, cash equivalents and restricted cash	(143,601)	350,973
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	343,019	486,062
Supplemental disclosure of cash flow information:
Cash paid for interest	6,365	1,243
Supplemental disclosure of noncash investing and financing activities:
Right-of-use assets obtained in exchange for lease liabilities	0	1,192
Change in unrealized gain (loss) on marketable securities, net of tax	(115)	318
Unpaid property and equipment	$ 0	$ 57