Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Activities
Net loss	$ (124,858)	$ (118,871)	$ (108,454)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	3,941	3,747	2,825
Stock-based compensation	22,995	24,569	28,248
Other non-cash items	128	409	1,168
Changes in assets and liabilities:
Prepaid expenses and other current assets	(2,429)	(2,986)	(675)
Collaboration receivables	(1,508)	(3,470)	(336)
Non-cash lease expense	5,143	0	0
Accounts payable	1,876	(673)	(504)
Accrued expenses	6,258	1,904	1,493
Operating lease obligations (Note 14)	(5,333)
Other changes in operating assets and liabilities	(17)	665	(309)
Net cash used in operating activities	(93,804)	(94,706)	(76,544)
Investing Activities
Purchase of investments	(395,138)	(73,570)	(179,935)
Proceeds from sale of securities	327,134	199,087	119,965
Purchases of property and equipment	(3,287)	(2,590)	(4,396)
Net cash (used in) provided by investing activities	(71,291)	122,927	(64,366)
Financing Activities
Proceeds from issuance of common stock from follow-on public offering, net of issuance costs	248,130	0	215,802
Payments for capital lease expenditures	0	(139)	0
Net proceeds from exercises and vesting of stock awards, ESPP contributions, and exercise of warrants to purchase common stock	10,590	16,285	4,494
Net cash provided by financing activities	258,720	16,146	220,296
Net increase in cash, cash equivalents and restricted cash	93,625	44,367	79,386
Cash, cash equivalents and restricted cash at beginning of period	145,649	101,282	21,896
Cash, cash equivalents and restricted cash at end of period	239,274	145,649	101,282
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Cashless exercise of warrants	0	797	0
Capitalized follow-on public offering costs included in accrued expenses	0	221	0
Purchase of property and equipment included in accounts payable and accrued expenses	$ 444	$ 1,159	$ 194