Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net loss	$ (10,670)	$ (10,068)
Adjustments to reconcile net loss to cash used in operating activities:
Noncash interest expense	139
Noncash rent expense	(15)	(16)
Amortization of premium on marketable securities	71	43
Depreciation	61	37
Stock-based compensation	784	486
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(45)	52
Accounts payable	310	(257)
Accrued expenses and other current liabilities	(2,254)	730
Net cash used in operating activities	(11,619)	(8,993)
Cash flows from investing activities:
Purchases of short-term investments	(12,722)	(24,161)
Proceeds from the maturities of short-term investments	19,869	11,833
Purchases of property and equipment	(53)	(15)
Net cash provided by (used in) investing activities	7,094	(12,343)
Cash flows from financing activities:
Net change in cash and cash equivalents	(4,525)	(21,336)
Cash and cash equivalents, beginning of period	29,798	80,042
Cash and cash equivalents, end of period	25,273	58,706
Supplemental disclosure of cash flow information:
Cash paid for interest	1,462
Supplemental disclosure of noncash investing and financing activities:
Net unrealized gain (loss) on marketable securities	$ (18)	$ 11