Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net loss	$ (15,001)	$ (52,739)	$ (107,029)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	9,567	6,219	3,775
Net loss (gain) on disposal of property and equipment	253	(127)	83
Impairment of property and equipment	232	7,600	65
Stock-based compensation	1,132	2,808	3,940
Non-cash interest expense	655	347	285
Amortization of premiums and discounts on investments	34	221	1,041
Changes in operating assets and liabilities:
Accounts receivable	(2,300)	(1,357)	19,665
Tenant improvement allowance receivable		5,833	(2,593)
Prepaid expenses and other current assets	(116)	1,503	3,179
Other noncurrent assets	96	50	31
Restricted cash	2,997	598	5
Accounts payable	(1,820)	(993)	(6,390)
Accrued expenses	(5,189)	(2,064)	(7,838)
Lease exit obligation	(5,206)	4,981
Deferred revenue	2,927	(17,623)	(1,293)
Other liabilities	78
Estimated settlement liability	4,000
Deferred rent	(10,569)	(9,505)	8,672
Net cash used in operating activities	(18,230)	(54,248)	(84,402)
Investing activities
Purchases of property and equipment	(22)	(12,942)	(3,668)
Purchases of marketable securities	(19,085)	(42,306)	(175,391)
Proceeds from maturities and sales of marketable securities	11,550	109,767	191,129
Proceeds from sale of property and equipment	1,241	191
Net cash (used in) provided by investing activities	(6,316)	54,710	12,070
Financing activities
Proceeds from issuance of common stock, net of issuance costs	10,218		53,637
Proceeds from issuance of stock for stock-based compensation arrangements	268	191	465
Proceeds from issuance of loans payable		10,000
Payments of debt issuance cost		(1,388)
Principal payments on loans payable	(11,612)	(7,785)	(7,104)
Net cash (used in) provided by financing activities	(1,126)	1,018	46,998
Net (decrease) increase in cash and cash equivalents	(25,672)	1,480	(25,334)
Effect of exchange rate changes on cash and cash equivalents			26
Cash and cash equivalents at beginning of period	52,306	50,826	76,134
Cash and cash equivalents at end of period	26,634	52,306	50,826
Supplemental cash flow and noncash investing and financing activities
Cash paid for interest	$ 1,983	2,018	$ 2,916
Non-cash financing activity
Fair value of warrants issued in connection with long-term debt		$ 413