Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net loss	$ (180,307)	$ (146,969)	$ (6,771)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	917	297
Stock-based compensation expense	20,819	30,947	32
In process research and development expenses	3,600	49,659
Amortization of operating lease right-of-use assets	2,172
Amortization of long-term debt discount and issuance costs	237
Amortization of premium on investments, net of accretion of discounts	(2,364)
Net realized gain on investments	(28)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(4,393)	(2,827)	(130)
Other assets	2,714	(584)
Operating lease liabilities	(2,108)
Accounts payable	(1,399)	2,085	57
Accrued expenses	1,521	5,938	824
Accrued research and development expenses	8,605	10,527	126
Accrued compensation and benefits	5,180
Accrued interest expense		(117)	117
Net cash used in operating activities	(144,834)	(51,044)	(5,745)
Cash flows from investing activities
Research and development asset acquisitions, net of cash acquired	(3,600)	(17,721)
Purchase of marketable securities	(499,079)	(123,500)
Maturities of marketable securities	328,000
Sales of marketable securities	30,501
Purchase of property and equipment	(2,966)	(3,490)
Net cash used in investing activities	(147,144)	(144,711)
Cash flows from financing activities
Proceeds from issuance of convertible note			6,000
Proceeds from issuance of common stock in a public offering, net	291,311
Proceeds from the issuance of long-term debt, net of debt discount and issuance costs of $1,778	28,222
Proceeds from the exercise of stock options	2,045
Repayment of notes payable to related parties		(40)
Payment of deferred offering costs		(2,144)
Net cash provided by financing activities	321,578	300,859	6,000
Effect of exchange rate changes on cash, cash equivalents and restricted cash	70
Net increase in cash, cash equivalents and restricted cash	29,600	105,104	255
Cash, cash equivalents and restricted cash, at the beginning of the period	105,419	315	60
Cash, cash equivalents and restricted cash, at the end of the period	135,089	105,419	$ 315
Supplemental disclosure of cash flow information:
Cash paid for interest	1,475	119
Supplemental disclosure of noncash investing and financing activities:
Acquisition of in-process research and development through issuance of stock		19,284
Issuance of Series A convertible preferred stock to convert debt and accrued interest		6,124
Unpaid deferred offering costs - net		1,545
Right-of-use assets obtained in exchange for lease liabilities	12,458
Conversion of convertible preferred stock to common stock	338,367
Change in unrealized gain on marketable securities, net of tax	331	(61)
Unpaid property and equipment	$ 183
Series A Convertible Preferred Stock
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net		73,491
Series B Convertible Preferred Stock
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net		$ 229,552