Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities:
Net loss	$ (25,543)	$ (31,470)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	400	457
Stock-based compensation	2,460	2,653
Accretion of debt discount	76	97
Amortization of premium of available-for-sale investment securities	113	397
Deferred rent	(130)	(154)
Changes in operating assets and liabilities
Prepaid expenses and other assets	706	394
Accounts payable and accrued expenses	(1,246)	1,147
Net cash used in operating activities	(23,164)	(26,479)
Cash flows from investing activities:
Purchases of property and equipment	(711)	(490)
Purchases of available-for-sale investment securities	(11,489)	(21,924)
Maturities of available-for-sale investment securities	27,850	40,651
Net cash provided by investing activities	15,650	18,237
Cash flows from financing activities:
Proceeds from issuance of common stock through option exercises	23	19
Proceeds through employee stock purchase plan	88	75
Proceeds from borrowing, net	4,930
Repayments on loans payable		(1,686)
Net cash provided by (used in) financing activities	5,041	(1,592)
Net change in cash and cash equivalents	(2,473)	(9,834)
Cash and cash equivalents at beginning of period	38,388	53,025
Cash and cash equivalents at the end of period	$ 35,915	$ 43,191