CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 70,154	$ 106,307
Marketable securities	21,980	50,996
Restricted cash	10,847	28,234
Prepaid expenses and other current assets	16,295	17,774
Total current assets	119,276	203,311
Restricted cash	958	890
Property and equipment, net	28,426	35,987
Intangible assets, net	6,541	7,873
Goodwill	51,967	53,563
Other non-current assets	810	869
Total assets	207,978	302,493
Current liabilities:
Accounts payable	9,301	5,613
Accrued expenses	19,550	29,610
Accrued interest	5,078	5,078
Deferred revenue	10,010	25,625
Other current liabilities	1,600	7,749
Total current liabilities	45,539	73,675
Deferred revenue	2,500	2,500
Convertible notes payable	319,187	317,763
Other non-current liabilities	8,687	10,287
Total liabilities	375,913	404,225
Commitments and contingencies
Stockholders' deficit:
Preferred stock, $0.01 par value, 2,000,000 shares authorized; no shares issued and outstanding at December 31, 2018 and 2017	0	0
Common stock, $0.01 par value, 600,000,000 shares authorized at December 31, 2018 and 2017; and 384,906,037 shares issued and 384,450,607 shares outstanding at December 31, 2018 and 323,684,820 shares issued and 323,229,390 shares outstanding at December 31, 2017	3,849	3,237
Additional paid-in capital	1,140,964	1,020,457
Accumulated deficit	(1,299,107)	(1,114,359)
Treasury stock, 455,430 shares, cost basis at both December 31, 2018 and 2017	(2,450)	(2,450)
Accumulated other comprehensive loss	(11,191)	(8,617)
Total stockholders' deficit	(167,935)	(101,732)
Total liabilities and stockholders' deficit	$ 207,978	$ 302,493