Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flow from operating activities:
Net loss	$ (36,233)	$ (30,223)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Stock-based compensation expense	8,413	2,746
Depreciation	723	634
Amortization of premiums on investments	(53)	41
Change in operating assets and liabilities:
Receivable from collaboration partner	1,013	45,000
Prepaid expenses and other current assets	(728)	152
Other long-term assets	(58)	(80)
Accounts payable	698	(488)
Accrued liabilities	1,388	1,470
Prepayment from collaboration partner	4,683
Other long-term liabilities	(88)	(86)
Deferred revenue	(11,970)	(5,411)
Net cash (used in) provided by operating activities	(32,212)	13,755
Cash flow from investing activities:
Purchases of investments	(39,595)	(36,022)
Sales of investments		4,000
Maturities of investments	12,000
Purchases of property and equipment	(2,356)	(617)
Net cash (used in) investing activities	(29,951)	(32,639)
Cash flow from financing activities:
Proceeds from issuance of common stock during follow-on offerings, net of issuance costs	182,450
Payments of financing-related costs	(381)	(38)
Proceeds from exercise of stock options and employee stock purchase plan	2,350	430
Net cash provided by financing activities	184,419	392
Net increase (decrease) in cash, cash equivalents and restricted cash	122,256	(18,492)
Cash, cash equivalents and restricted cash, beginning of period	224,857	136,056
Cash, cash equivalents and restricted cash, end of period	347,113	117,564
Non-cash investing and financing activities:
Vesting of early exercised options and restricted stock	32	99
Unpaid financing-related costs	206
Unpaid portion of property and equipment purchases included in period-end accounts payable and accrued liabilities	$ 386	$ 218