Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities
Net loss	$ (32,611)	$ (26,037)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	169	6
Stock-based compensation expense	3,089	605
In process research and development expenses	1,000	20,898
Changes in operating assets and liabilities:
Operating lease right of use assets and liabilities, net	61
Prepaid expenses and other current assets	(3,472)	(94)
Other Assets	2,144	(425)
Accounts payable	2,132	546
Accrued expenses	(789)	441
Accrued research and development expenses	2,072	(126)
Accrued compensation and benefits	(1,569)	593
Accrued interest expense		(115)
Net cash used in operating activities	(27,774)	(3,708)
Cash flows from investing activities
Research and development asset acquisitions, net of cash acquired	(1,000)	11,176
Purchase of investments	(222,295)
Sales and maturities of investments	25,500
Purchase of property and equipment	(347)	(511)
Net cash (used in) provided by investing activities	(198,142)	10,665
Cash flows from financing activities
Proceeds from issuance of common stock in a public offering, net	291,273
Proceeds from the exercise of stock options	71
Proceeds from issuance of Series A convertible preferred stock, net		71,944
Repayment of notes payable to related parties		(40)
Net cash provided by financing activities	291,344	71,904
Net increase in cash, cash equivalents and restricted cash	65,428	78,861
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	170,847	79,176
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	140
Unpaid property and equipment	$ 739	$ 43