Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities
Net loss	$ (116,107)	$ (73,684)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	190	161
Stock-based compensation expense	11,101	8,028
Changes in assets and liabilities
Prepaid expenses and other assets	(93)	(1,539)
Accounts payable	2,876	29
Accrued and other liabilities	22,995	5,007
Income taxes payable	(247)
Upfront payment from collaboration partner		40,000
Net cash used in operating activities	(79,285)	(21,998)
Cash flows from investing activities
Purchase of property and equipment	(3,532)	(90)
Change in restricted cash	(2,382)
Net cash used in investing activities	(5,914)	(90)
Cash flows from financing activities
Proceeds from issuance of common stock in connection with employee benefit plans	2,829	453
Net cash provided by financing activities	2,829	453
Net increase (decrease) in cash and cash equivalents	(82,370)	(21,635)
Cash and cash equivalents at beginning of period	201,153	182,069
Cash and cash equivalents at end of period	118,783	160,434
Supplemental disclosure
Income taxes paid	375
Supplemental disclosure of noncash items
Build-to-suit lease transaction	$ 5,428
Public offering issuance costs		$ 346