Statements of Cash Flows (USD $)	3 Months Ended		85 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014
Operating activities:
Net loss	$ (9,697,110)	$ (620,914)	$ (72,739,027)
Adjustments to reconcile net loss to net cash used in operating activities:
Gain on extinguishment of debt and other liabilities		(2,419,766)	(3,735,499)
Depreciation expense	8,118	0	99,539
Amortization of debt issuance costs		751,880	81,248
Amortization of debt discount and interest expense			5,159,034
Gain on cancellation of preferred stock future tranche rights			(653,465)
Issuance of 2007 Series X preferred stock for licensing agreement			31,800
Stock-based compensation expense	2,444,864	56,392	4,754,116
Changes in operating assets and liabilities:
Accounts receivable	15,944	(13,828)	(119,395)
Prepaid expenses and other current assets	(719,534)	264,212	(1,429,607)
Other assets			(125,345)
Accounts payable and accrued expenses	1,909,614	452,445	5,526,503
Deferred rent	30,434		30,434
Net cash used in operating activities	(6,007,670)	(1,529,579)	(63,119,664)
Investing activities:
Purchases of property and equipment	(141,956)		(414,747)
Proceeds from maturities of short-term investments	5,500,000		18,677,943
Proceeds from sale of short-term investments			800,234
Purchases of short-term investments			(25,383,162)
Net cash (used in) provided by investing activities	5,358,044		(6,319,732)
Financing activities:
Proceeds from issuance of 2012 Series X preferred stock			83,033,938
Preferred stock issuance costs		(17,348)	(1,682,007)
Debt issuance costs			(76,857)
Initial public offering issuance costs	(9,199,504)		(9,420,672)
Proceeds from issuance of common stock	114,954,000		114,959,376
Proceeds from issuance of convertible debt			3,945,715
Payments on capital lease obligations	(641)		(641)
Net cash provided by financing activities	105,753,855	2,482,659	195,758,853
Increase (decrease) in cash and cash equivalents	105,104,229	953,080	126,319,457
Cash and cash equivalents at beginning of period	21,215,228	1,641,038
Cash and cash equivalents at end of period	126,319,457	2,594,118	126,319,457
Non-cash financing activities:
Issuance of Series A preferred stock in settlement of convertible notes			5,383,000
Accretion of preferred stock to redemption value		848,125	153,637,754
Reclassification of 2012 Series X preferred stock from debt to preferred stock			2,486,251
Conversion of 2012 Series X preferred stock into Series C preferred stock			4,944,155
Unpaid initial public offering issuance costs	1,168,768		1,168,768
Assets acquired under capital lease			11,916
Loss on extinguishment of 2007 Series X preferred stock			597,174
Book value of assets transferred in distribution of subsidiary			193,114
Book value of liabilities transferred in distribution of subsidiary			(575,964)
Exchange of convertible debt			304,054
Series X Preferred Stock [Member]
Financing activities:
Proceeds from issuance of 2012 Series X preferred stock		$ 2,500,007	$ 5,000,001