CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 88,475	$ 106,307
Marketable securities	78,636	50,996
Restricted cash	10,132	28,234
Prepaid expenses and other current assets	17,092	17,774
Total current assets	194,335	203,311
Restricted cash	952	890
Property and equipment, net	30,829	35,987
Intangible assets, net	6,897	7,873
Goodwill	51,983	53,563
Other non-current assets	811	869
Total assets	285,807	302,493
Current liabilities:
Accounts payable	6,686	5,613
Accrued expenses	22,042	29,610
Accrued interest	5,078	5,078
Deferred revenue	7,683	25,625
Other current liabilities	1,530	7,749
Total current liabilities	43,019	73,675
Deferred revenue	2,500	2,500
Convertible notes payable	318,475	317,763
Other non-current liabilities	9,520	10,287
Total liabilities	373,514	404,225
Commitments and contingencies	0	0
Stockholders' deficit:
Preferred stock, $0.01 par value, 2,000,000 shares authorized; no shares issued and outstanding as of June 30, 2018 and December 31, 2017, respectively	0	0
Common stock, $0.01 par value, 600,000,000 shares authorized at June 30, 2018 and December 31, 2017; 382,139,415 shares issued and 381,683,985 shares outstanding at June 30, 2018 and 323,684,820 shares issued and 323,229,390 shares outstanding at December 31, 2017	3,821	3,237
Additional paid-in capital	1,127,298	1,020,457
Accumulated deficit	(1,205,203)	(1,114,359)
Treasury stock, 455,430 shares, cost basis at both June 30, 2018 and December 31, 2017	(2,450)	(2,450)
Accumulated other comprehensive loss	(11,173)	(8,617)
Total stockholders' deficit	(87,707)	(101,732)
Total liabilities and stockholders' deficit	$ 285,807	$ 302,493