Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities
Net loss	$ (158,987)	$ (94,495)	$ (33,811)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	23,020	15,240	2,512
Non-cash licensing and consulting fees		1,211	127
Premium on marketable securities	(756)
Amortization of premium on marketable securities	286
Depreciation	281	115	51
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,362)	(681)	(715)
Accounts payable	7,796	2,590	441
Accrued expenses and other liabilities	13,044	5,339	4,353
Net cash used in operating activities	(118,678)	(70,681)	(27,042)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	30,499
Purchases of marketable securities	(259,093)
Purchases of property and equipment	(1,421)	(198)	(128)
Increase in restricted cash	(525)
Net cash used in investing activities	(230,540)	(198)	(128)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	1,406	730	40
Payments of offering costs	(599)	(584)	(2,285)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts	330,175	129,720	96,255
Net cash provided by financing activities	330,982	129,866	146,870
Net increase (decrease) in cash and cash equivalents	(18,236)	58,987	119,700
Cash and cash equivalents at beginning of period	186,753	127,766	8,066
Cash and cash equivalents at end of period	168,517	186,753	127,766
Supplemental disclosure of non-cash financing activities
Accretion of redeemable convertible preferred stock to redemption value			2,294
Conversion of preferred stock to common stock			92,863
Purchases of property and equipment included in accounts payable	$ 8
Public offering costs included in accounts payable or accrued expenses		$ 165
Series A Preferred Stock [Member]
Cash flows from financing activities
Proceeds from the issuance of preferred stock, net of issuance costs			14,970
Series B Preferred Stock [Member]
Cash flows from financing activities
Proceeds from the issuance of preferred stock, net of issuance costs			$ 37,890