Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities
Net loss	$ (56,778)	$ (30,543)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,208	3,714
Amortization of premium on marketable securities	33
Depreciation	130	41
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,380)	(1,189)
Accounts payable	(4,643)	(799)
Accrued expenses and other liabilities	1,220	1,341
Net cash used in operating activities	(55,210)	(27,435)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	50,753
Purchases of marketable securities	(18,908)
Purchases of property and equipment	(245)	(341)
Net cash provided by (used in) investing activities	31,600	(341)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	553	302
Payments of offering costs		(599)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts		141,000
Net cash provided by financing activities	553	140,703
Net increase (decrease) in cash and cash equivalents	(23,057)	112,927
Cash and cash equivalents at beginning of period	168,517	186,753
Cash and cash equivalents at end of period	145,460	299,680
Supplemental disclosure of non-cash financing activities
Purchases of property and equipment included in accounts payable or accrued expenses	$ 64	$ 279