Consolidated Balance Sheets - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 306,235,000	$ 168,517,000
Marketable securities	212,613,000	228,962,000
Prepaid expenses and other current assets	6,227,000	5,100,000
Total current assets	525,075,000	402,579,000
Property and equipment, net	4,013,000	1,388,000
Restricted cash	849,000	564,000
Total assets	529,937,000	404,531,000
Current liabilities:
Accounts payable	9,350,000	12,817,000
Accrued expenses	42,601,000	22,352,000
Total current liabilities	51,951,000	35,169,000
Other liabilities	2,511,000	845,000
Total liabilities	54,462,000	36,014,000
Commitments and contingencies (Note 5)
Stockholders’ equity:
Preferred stock, $0.0001 par value per share; 5,000,000 shares authorized at December 31, 2017 and 2016; no shares issued or outstanding at December 31, 2017 and 2016
Common stock, $0.0001 par value per share; 120,000,000 shares authorized at December 31, 2017 and 2016; 42,003,894 and 37,222,518 shares issued at December 31, 2017 and 2016, respectively; 42,002,934 and 37,222,172 shares outstanding at December 31, 2017 and 2016, respectively	5,000	4,000
Treasury stock, at cost, 960 and 346 shares at December 31, 2017 and 2016, respectively	(113,000)	(17,000)
Additional paid-in capital	1,066,059,000	688,959,000
Accumulated deficit	(590,447,000)	(320,327,000)
Accumulated other comprehensive loss	(29,000)	(102,000)
Total stockholders’ equity	475,475,000	368,517,000
Total liabilities and stockholders’ equity	$ 529,937,000	$ 404,531,000