Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Cash flows from operating activities
Net loss	$ (65,290)	$ (41,898)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	8,179	6,747
Non-cash licensing and consulting fees		1,211
Premium on marketable securities	(269)
Amortization of premium on marketable securities	13
Depreciation	116	54
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(74)	(2,219)
Accounts payable	941	2,760
Accrued expenses and other liabilities	2,278	296
Net cash used in operating activities	(54,106)	(33,049)
Cash flows from investing activities
Purchases of marketable securities	(82,997)
Purchases of property and equipment	(835)	(181)
Increase in restricted cash	(525)
Net cash used in investing activities	(84,357)	(181)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	312	436
Payments of offering costs	(599)	(458)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts	141,000	129,720
Net cash provided by financing activities	140,713	129,698
Net increase in cash and cash equivalents	2,250	96,468
Cash and cash equivalents at beginning of period	186,753	127,766
Cash and cash equivalents at end of period	$ 189,003	224,234
Supplemental disclosure of non-cash financing activities
Public offering costs included in accounts payable or accrued expenses		$ 137