Balance Sheets (USD $)	Mar. 31, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 126,319,457	$ 21,215,228
Investments	5,831,843	11,341,241
Accounts receivable	119,395	135,339
Prepaid expenses and other current assets	1,468,165	739,235
Total current assets	133,738,860	33,431,043
Equipment, net	164,204	30,366
Deferred offering costs		1,078,138
Other assets	125,345	125,345
Total assets	134,028,409	34,664,892
Current liabilities:
Accounts payable	2,077,302	714,137
Accrued expenses	4,042,007	3,183,761
Current portion of capital lease obligation	3,911	3,912
Total current liabilities	6,123,220	3,901,810
Capital lease obligation, net of current portion	7,364	8,004
Other liabilities	30,434
Total liabilities	6,161,018	3,909,814
Redeemable convertible preferred stock; $0.00001 par value; 0 and 5,500,636 shares authorized at March 31, 2014 and December 31, 2013, respectively:
Redeemable convertible preferred stock		157,827,135
Stockholders' equity (deficit):
Common stock; $0.00001 par value; 175,000,000 and 14,700,000 authorized at March 31, 2014 and December 31, 2013, respectively; 20,262,693 and 1,383,345 shares issued and outstanding at March 31, 2014 and December 31, 2013, respectively	202	14
Additional paid-in capital	201,203,384
Deficit accumulated during the development stage	(73,336,195)	(127,072,071)
Total stockholders' equity (deficit)	127,867,391	(127,072,057)
Total liabilities, redeemable convertible preferred stock and stockholders' equity (deficit)	134,028,409	34,664,892
Series A redeemable convertible preferred stock [Member]
Redeemable convertible preferred stock; $0.00001 par value; 0 and 5,500,636 shares authorized at March 31, 2014 and December 31, 2013, respectively:
Redeemable convertible preferred stock		39,367,094
Series B redeemable convertible preferred stock [Member]
Redeemable convertible preferred stock; $0.00001 par value; 0 and 5,500,636 shares authorized at March 31, 2014 and December 31, 2013, respectively:
Redeemable convertible preferred stock		21,257,044
Series C redeemable convertible preferred stock [Member]
Redeemable convertible preferred stock; $0.00001 par value; 0 and 5,500,636 shares authorized at March 31, 2014 and December 31, 2013, respectively:
Redeemable convertible preferred stock		$ 97,202,997