Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities
Net loss	$ (126,979)	$ (65,290)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	15,558	8,179
Premium on marketable securities		(269)
Amortization of premium on marketable securities	5	13
Depreciation	260	116
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(93)	(74)
Accounts payable	(6,768)	941
Accrued expenses and other liabilities	3,731	2,278
Net cash used in operating activities	(114,286)	(54,106)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	110,436
Purchases of marketable securities	(33,922)	(82,997)
Purchases of property and equipment	(321)	(835)
Increase in restricted cash	285	525
Net cash provided by (used in) investing activities	75,908	(84,357)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	3,311	312
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	3,311	140,713
Net increase (decrease) in cash and cash equivalents	(35,067)	2,250
Cash and cash equivalents at beginning of period	168,517	186,753
Cash and cash equivalents at end of period	$ 133,450	$ 189,003