Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (29,497)	$ (42,854)
Adjustments to reconcile net loss to cash used in operating activities:
Noncash interest expense	581	222
Noncash rent	(59)	(60)
Noncash asset impairment charge	437
Amortization of premium on marketable securities	217	225
Depreciation	200	169
Stock-based compensation	4,036	2,909
Changes in operating assets and liabilities:
Prepaid expenses and other assets	960	(948)
Accounts payable	(1,141)	(41)
Accrued expenses and other liabilities	(1,846)	3,112
Net cash used in operating activities	(26,112)	(37,266)
Cash flows from investing activities:
Purchases of short-term investments	(27,204)	(122,277)
Proceeds from the maturities of short-term investments	102,289	56,705
Purchase of property and equipment	(70)	(487)
Net cash provided by (used in) investing activities	75,015	(66,059)
Cash flows from financing activities:
Net proceeds from issuance of common stock		3,997
Payments made for taxes of employees who surrendered shares related to settlement of restricted stock	(16)
Net cash provided by financing activities:	19	53,081
Net change in cash and cash equivalents	48,922	(50,244)
Cash and cash equivalents, beginning of period	29,798	80,042
Cash and cash equivalents, end of period	78,720	29,798
Supplemental disclosure of cash flow information:
Cash paid for interest	2,957
Supplemental disclosure of noncash investing and financing activities:
Net unrealized gain (loss) on marketable securities	38	(43)
2021 Convertible Notes [Member]
Cash flows from financing activities:
Proceeds from issuance of 2021 Convertible Notes		52,000
Payments of 2021 Convertible Notes issuance costs		(3,262)
Stock Option Plans [Member]
Cash flows from financing activities:
Proceeds from the issuance of common stock		191
Employee Stock Purchase Plan [Member]
Cash flows from financing activities:
Proceeds from the issuance of common stock	$ 35	$ 155