Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities
Net loss	$ (143,551)	$ (97,165)	$ (201,810)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	547	319	265
Accretion of premiums and discounts on investments	(97)	(200)	(217)
Amortization of discount and issuance costs on convertible notes	1,741	0	0
Acquired in-process research and development (Note 3)	12,500	0	0
Non-cash interest expense related to the revenue interest liability	19,560	8,120	0
Stock-based compensation expense	28,385	25,884	23,968
Changes in assets and liabilities:
Prepaids and other assets	(14,793)	(3,396)	(2,884)
Deferred revenue	(490)	2,152	0
Inventories	(16,136)	0	0
Other long-term liabilities	1,290	0	0
Accounts payable	23,106	(16,055)	24,446
Other accrued liabilities	2,761	10,000	7,594
Net cash used in operating activities	(85,177)	(70,341)	(148,638)
Investing activities
Purchases of investments	(4,420)	(34,326)	(25,481)
Purchases of in-process research and development	(12,500)	0	0
Proceeds from sales/maturities of investments	39,145	99,510	166,081
Purchase of property and equipment	(869)	(953)	(151)
Net cash provided by investing activities	21,356	64,231	140,449
Financing activities
Proceeds from issuance of convertible notes	280,000	0	0
Payment for debt issuance costs	(8,405)	0	0
Prepayment of forward stock repurchase transaction	(55,000)	0	0
Purchase of capped call options associated with convertible notes	(46,004)	0	0
Proceeds from revenue interest liability, net of issuance costs	25,000	124,424	0
Payments on revenue interest liability	(500)	0	0
Proceeds from exercise of common stock options	6,634	11,771	11,743
Payments on long-term debt	0	0	(1,049)
Net cash provided by financing activities	201,725	136,195	10,694
Net increase in cash, cash equivalents and restricted cash	137,904	130,085	2,505
Cash and cash equivalents at beginning of period	167,058	36,973	34,468
Cash, cash equivalents and restricted cash at end of period	304,962	167,058	36,973
Supplemental disclosure of cash flow information:
Purchase of property and equipment not yet paid	0	190	199
Non cash right of use asset	20	31	0
Offering costs not yet paid	$ 495	$ 0	$ 0