Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Operating Activities
Net loss		$ (50,939)	$ (38,053)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization		996	933
Stock-based compensation		6,679	6,992
Other non-cash items		1,365	959
Changes in assets and liabilities:
Prepaid expenses and other assets		828	(237)
Collaboration receivables (all amounts are with a related party)		112	4,259
Non-cash lease expense	[1]	1,377	1,234
Accounts payable		1,634	3,466
Accrued expenses		(7,148)	(4,544)
Operating lease obligations		(1,466)	(1,317)
Other changes in operating assets and liabilities		17	0
Net cash used in operating activities		(46,545)	(26,308)
Investing Activities
Purchases of investments		(10,454)	(216,116)
Proceeds from sales and maturities of investments		88,361	64,133
Purchases of property and equipment		(426)	(587)
Net cash provided by (used in) investing activities		77,481	(152,570)
Financing Activities
Proceeds from issuance of common stock from public offering, net of issuance costs		0	248,155
Net proceeds from exercises and vesting of stock awards, ESPP contributions, and exercise of warrants to purchase common stock		8,873	1,161
Net cash provided by financing activities		8,873	249,316
Net increase in cash, cash equivalents and restricted cash		39,809	70,438
Cash, cash equivalents and restricted cash at beginning of period		239,274	145,649
Cash, cash equivalents and restricted cash at end of period		279,083	216,087
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Purchase of property and equipment included in accounts payable and accrued expenses		$ 425	$ 267

[1] The Company has reclassified prior period amounts to conform with presentation as of March 31, 2020.