Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net loss	$ (270,120)	$ (158,987)	$ (94,495)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	35,534	23,020	15,240
Non-cash licensing and consulting fees			1,211
Premium on marketable securities	(105)	(756)
Amortization of premium (discount) on marketable securities	(293)	286
Depreciation	531	281	115
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,127)	(3,362)	(681)
Accounts payable	(3,693)	7,796	2,590
Accrued expenses and other liabilities	19,996	13,044	5,339
Net cash used in operating activities	(219,277)	(118,678)	(70,681)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	261,225	30,499
Purchases of marketable securities	(244,403)	(259,093)
Purchases of property and equipment	(1,360)	(1,421)	(198)
Increase in restricted cash	(285)	(525)
Net cash provided by (used in) investing activities	15,177	(230,540)	(198)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	15,972	1,406	730
Payments of offering costs	(179)	(599)	(584)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts	326,025	330,175	129,720
Net cash provided by financing activities	341,818	330,982	129,866
Net increase (decrease) in cash and cash equivalents	137,718	(18,236)	58,987
Cash and cash equivalents at beginning of period	168,517	186,753	127,766
Cash and cash equivalents at end of period	306,235	168,517	186,753
Supplemental disclosure of non-cash financing activities
Purchases of property and equipment financed with landlord tenant incentive	1,665
Purchases of property and equipment included in accounts payable	138	$ 8
Public offering costs included in accounts payable or accrued expenses	$ 95		$ 165