CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities:
Net loss	$ (209,383)	$ (419,503)	$ (123,914)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	29,176	21,247	15,492
Depreciation and amortization	5,465	1,268	155
Deferred income taxes	(17,425)	0	0
Noncash lease expense	1,137	1,221
Amortization of debt discount and issuance costs	2,143	0	0
Net accretion and amortization of investments in marketable securities	(570)	(4,887)	(1,998)
Acquired in-process research and development (IPR&D)	10,700	251,437	0
Change in fair value of contingent consideration	8,600	5,600	1,300
Other	(205)	(471)	(169)
Changes in operating assets and liabilities:
Accounts receivable	(3,824)	0	0
Inventory	(1,026)	0	0
Prepaid expenses and other current assets	(575)	7,573	(9,335)
Other assets	(1,613)	(5,723)	266
Accounts payable, accrued and other current liabilities	13,711	5,647	6,545
Operating lease liability	(1,287)	11,720
Deferred revenue	(2,555)	13,352	0
Net cash used in operating activities	(167,531)	(111,519)	(111,658)
Cash flows from investing activities:
Cash paid for IPR&D assets	(10,700)	(179,624)	0
Purchase of note receivable	(5,000)	0	0
Purchases of marketable securities	(509,335)	(329,641)	(569,515)
Proceeds from maturities of marketable securities	12,987	415,020	125,783
Proceeds from sale of marketable securities	347,627	176,858	0
Purchases of property and equipment	(679)	(9,492)	(1,018)
Net cash (used in) provided by investing activities	(165,100)	73,121	(444,750)
Cash flows from financing activities:
Payments of contingent consideration amounts previously established in purchase accounting	(15,000)	(20,000)	0
Proceeds from issuance of common stock under equity incentive plans	5,515	10,182	9,654
Taxes paid related to net share settlement of equity awards	(2,157)	(744)	(1,430)
Net proceeds from issuance of Convertible Senior Notes	223,100	0	0
Payment of debt issuance costs	(557)	0	0
Proceeds from issuance of common stock, net	226,576	42,576	323,135
Net cash provided by financing activities	437,477	32,014	331,359
Net increase (decrease) in cash and cash equivalents	104,846	(6,384)	(225,049)
Cash and cash equivalents at beginning of period	62,070	68,454	293,503
Cash and cash equivalents at end of period	166,916	62,070	68,454
Noncash investing and financing activities:
Common stock issued as consideration for asset acquisition	0	68,124	0
Net liabilities assumed in connection with asset acquisition	0	3,688	0
Purchases of property and equipment in accounts payable	$ 0	$ 0	$ 1,762