Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating activities
Net loss	$ (29,917)	$ (27,059)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	183	179
Net amortization of premiums and discounts on investments	267	314
Stock-based compensation expense	5,909	5,158
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(61)	243
Accounts payable	(522)	294
Accrued expenses and other liabilities	(498)	(2,282)
Deferred rent	(69)	(12)
Net cash used in operating activities	(24,708)	(23,165)
Investing activities
Purchases of property and equipment		(39)
Proceeds from maturities of investments	25,624	65,793
Purchases of investments	(25,075)	(59,003)
Net cash provided by investing activities	549	6,751
Financing activities
Proceeds from the exercise of stock options	57	196
Net cash provided by financing activities	57	196
Effect of exchange rate on cash	16	48
Net decrease in cash and cash equivalents	(24,086)	(16,170)
Cash and cash equivalents at beginning of period	49,663	58,358
Cash and cash equivalents at end of period	25,577	$ 42,188
Supplemental disclosure of non-cash financing activity
Deferred financing costs included in accounts payable	$ 15