Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net loss	$ (125,609)	$ (108,175)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	625	163
Stock-based compensation expense	13,957	20,148
In process research and development expenses	2,000	49,659
Amortization of long-term debt discount and issuance costs	148
Amortization of premium on investments, net of accretion of discounts	(2,296)
Net realized gain on investments	(1)
Changes in operating assets and liabilities:
Operating lease right of use assets and liabilities, net	49
Prepaid expenses and other current assets	(2,807)	(1,442)
Other assets	2,886	(823)
Accounts payable	333	3,525
Accrued expenses	(760)	3,263
Accrued research and development expenses	8,518	7,778
Accrued compensation and benefits	2,263
Accrued interest expense		(117)
Net cash used in operating activities	(100,694)	(26,021)
Cash flows from investing activities
Research and development asset acquisitions, net of cash acquired	(2,000)	(17,721)
Purchase of investments	(399,393)	(115,141)
Maturities of investments	205,750
Sales of investments	4,004
Purchase of property and equipment	(2,438)	(3,168)
Net cash used in investing activities	(194,077)	(136,030)
Cash flows from financing activities
Proceeds from issuance of common stock in a public offering, net	291,311
Proceeds from the issuance of long-term debt, net of issuance costs of $1,778	28,222
Proceeds from the exercise of stock options	527
Repayment of notes payable to related parties		(40)
Net cash provided by financing activities	320,060	303,003
Net increase in cash, cash equivalents and restricted cash	25,289	140,952
Cash, cash equivalents and restricted cash, at the beginning of the period	105,419	315
Cash, cash equivalents and restricted cash, at the end of the period	130,708	141,267
Supplemental disclosure of cash flow information:
Cash paid for interest	854
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
Recognition of operating lease right of use asset	12,458
Recognition of operating lease liabilities	13,182
Conversion of convertible preferred stock to common stock	338,367
Change in unrealized gain on marketable securities, net of tax	510	235
Change in unrealized loss on foreign currency translations, net of tax	121
Unpaid property and equipment	$ 127
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