Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Operating activities
Net loss	$ (212,480)	$ (150,687)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	39,622	24,264
Depreciation and amortization expense	5,174	2,254
Accretion of investment discounts	(1,177)	(1,339)
Loss on disposals of property and equipment	927	0
Non-cash operating lease expense	768	0
Non-cash interest expense	0	211
Asset retirement obligation accretion expense	53	32
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,204	(2,339)
Operating lease assets	238	0
Other assets	308	(225)
Accounts payable	1,527	116
Accrued compensation	2,261	3,408
Accrued research and development expenses	(13,038)	(203)
Other current liabilities	(1,836)	2,063
Operating lease liabilities	(487)	0
Other long-term liabilities	0	92
Net cash used in operating activities	(176,936)	(122,353)
Investing activities
Purchases of short-term investments	(147,187)	(402,621)
Proceeds from maturities and sales of short-term investments	209,379	192,020
Purchases of property and equipment	(3,021)	(31,756)
Proceeds from sale of property and equipment	161	0
Net cash provided by (used in) investing activities	59,332	(242,357)
Financing activities
Proceeds from sale of common stock and pre-funded warrants in underwritten offerings, net	140,888	293,290
Proceeds from issuance of common stock through ATM facility, net	12,630	47,586
Proceeds from employee stock awards	4,859	18,528
Taxes paid related to net share settlement of restricted stock units	(6,695)	(7,493)
Principal payments on finance and capital lease obligations	(345)	(396)
Net cash provided by financing activities	151,337	351,515
Increase (decrease) in cash, cash equivalents and restricted cash	33,733	(13,195)
Cash, cash equivalents and restricted cash at beginning of period	62,092	80,617
Cash, cash equivalents and restricted cash at end of period	95,825	67,422
Non-cash investing and financing activities
Property and equipment purchases included in accounts payable and other accrued liabilities	699	3,973
Capitalized lease obligations	0	441
Property & equipment acquired under capital leases	0	191
Asset retirement cost	0	88
Interest capitalized during construction period for build-to-suit lease transaction	0	77
Supplemental cash flow disclosure
Cash paid for interest	$ 43	$ 110