Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net loss	$ (76,333)	$ (84,979)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	1,060	311
Write-off of acquired IPR&D	38,313
Stock-based compensation expense	16,139	13,349
Changes in assets and liabilities, net of acquisition
Prepaid expenses and other assets	(224)	3,940
Accounts payable	(1,744)	142
Deferred revenue	(1,371)
Accrued liabilities	(5,097)	(8,805)
Income taxes payable		(247)
Upfront payment from collaboration partner		(40,000)
Net cash used in operating activities	(29,257)	(116,289)
Cash flows from investing activities
Purchase of property and equipment	(33)	(745)
Leasehold improvements for build-to-suit asset		(3,558)
Cash paid to acquire IPR&D	(2,076)
Cash acquired in the merger transaction	5,277
Net cash provided by (used in) investing activities	3,168	(4,303)
Cash flows from financing activities
Inducement on build-to-suit lease obligation	1,896
Proceeds from issuance of common stock	52	2,968
Net cash provided by financing activities	1,948	2,968
Net decrease in cash, cash equivalents and restricted cash	(24,141)	(117,624)
Cash, cash equivalents and restricted cash at beginning of period	83,529	201,153
Cash, cash equivalents and restricted cash at end of period	59,388	83,529
Supplemental disclosure of noncash items
Build-to-suit lease transaction		$ 5,428
Issuance of common stock for the merger transaction	$ 37,331