Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities
Net loss	$ (120,944)	$ (77,109)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	679	374
Stock-based compensation expense	17,144	8,229
In process research and development expenses	17,805	2,000
Amortization of operating lease right-of-use assets	1,203	1,080
Amortization of debt discount and issuance costs	823	59
Amortization of premium on investments, net of accretion of discounts	(33)	(1,464)
Net realized gain on investments	(253)	(1)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(548)	(4,243)
Other assets	932	2,906
Operating lease liabilities	(715)	(1,032)
Accounts payable	(727)	823
Accrued expenses	(982)	(1,018)
Accrued research and development expenses	(6,103)	4,624
Accrued compensation and benefits	(2,975)	188
Net cash used in operating activities	(94,694)	(64,584)
Cash flows from investing activities
Research and development asset acquisitions, net of cash acquired	(17,805)	(2,000)
Purchase of marketable securities	(73,777)	(287,038)
Maturities of marketable securities	143,304	74,897
Sales of marketable securities	83,515	3,842
Purchase of property and equipment	(954)	(1,727)
Net cash provided by (used in) investing activities	134,283	(212,026)
Cash flows from financing activities
Proceeds from issuance of common stock	117,094	291,311
Proceeds from issuance of convertible debt, net	193,596
Proceeds from the issuance of long-term debt, net of issuance costs of $1,778		28,222
Proceeds from the exercise of stock options	154	158
Net cash provided by financing activities	311,400	319,691
Effect of exchange rate changes on cash and cash equivalents	(16)
Net increase in cash and cash equivalents	350,989	43,081
Cash and cash equivalents, at the beginning of the period	135,089	105,419
Cash and cash equivalents, at the end of the period	486,062	148,500
Supplemental disclosure of cash flow information:
Cash paid for interest	1,243
Supplemental disclosure of noncash investing and financing activities:
Right-of-use assets obtained in exchange for lease liabilities	1,192	12,458
Change in unrealized gain on marketable securities, net of tax	318	565
Unpaid property and equipment	57	268
Conversion of convertible preferred stock to common stock		$ 338,367
Employee Stock Purchase Plan
Cash flows from financing activities
Proceeds from issuance of common stock	$ 556