Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash flows from operating activities
Net loss	$ (103,086)	$ (65,933)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	12,927	11,154
Non-cash licensing and consulting fees		1,211
Premium on marketable securities	(663)
Amortization of premium on marketable securities	87
Depreciation	199	83
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,680)	(1,548)
Accounts payable	116	808
Accrued expenses and other liabilities	7,198	1,661
Net cash used in operating activities	(84,902)	(52,564)
Cash flows from investing activities
Proceeds from sales of marketable securities	7,999
Purchases of marketable securities	(118,650)
Purchases of property and equipment	(901)	(160)
Increase in restricted cash	(525)
Net cash used in investing activities	(112,077)	(160)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	728	663
Payments of offering costs	(599)	(548)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts	330,175	129,720
Net cash provided by financing activities	330,304	129,835
Net increase in cash and cash equivalents	133,325	77,111
Cash and cash equivalents at beginning of period	186,753	127,766
Cash and cash equivalents at end of period	320,078	204,877
Supplemental disclosure of non-cash financing activities
Public offering costs included in accounts payable or accrued expenses		$ 4
Purchases of property and equipment included in accounts payable or accrued expenses	$ 106