Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Operating activities
Net loss	$ (7,710)	$ (10,874)
Adjustments to reconcile net loss to net cash used in operating activities:
Impairment of property and equipment		232
Depreciation and amortization	5	4,488
Accretion		199
Loss on disposal of fixed assets		(20)
Stock-based compensation	390	427
Non-cash interest expense	86	125
Amortization of premium and discount on investments	3	14
Changes in operating assets and liabilities:
Restricted cash	(4,000)	37
Accounts receivable	2,905	1,394
Prepaid expenses and other current assets	492	337
Other noncurrent assets	19	26
Accounts payable	(1,085)	(626)
Accrued expenses	(1,263)	(2,900)
Deferred revenue	(203)	(76)
Lease exit obligation		(3,345)
Deferred rent		(5,200)
Other liabilities	42	(58)
Net cash used in operating activities	(10,319)	(15,820)
Investing activities
Purchases of marketable securities	(5,746)	(6,048)
Proceeds from maturities and sales of marketable securities	7,500	1,500
Proceeds from sale of property and equipment		928
Net cash provided by (used in) investing activities	1,754	(3,620)
Financing activities
Proceeds from issuance of common stock, net of issuance costs		4,353
Proceeds from exercise of stock options and issuance of common and restricted stock	2	73
Debt issuance costs	(15)
Principal payments on loans payable		(2,757)
Net cash (used by) provided by financing activities	(13)	1,669
Net decrease in cash and cash equivalents	(8,578)	(17,771)
Cash and cash equivalents at beginning of period	26,634	52,306
Cash and cash equivalents at end of period	18,056	34,535
Supplemental cash flow information
Cash paid for interest	$ 301	$ 617