Organization and Nature of Business (Details) (USD $)	12 Months Ended			1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	May 31, 2014	Nov. 30, 2014	Dec. 31, 2011
Capital Resources and Liquidity
Net loss	$ 48,429,000	$ 25,056,000	$ 74,495,000
Accumulated deficit	210,989,000	162,560,000
Cash and cash equivalents	147,539,000	43,819,000	22,062,000			11,855,000
Common stock
Public Offering
Shares sold	8,941,250
Shares sold in public follow-on offering (in shares)	1,960,390
Shares of common stock issued for conversion of Series A preferred				2,857,143
Common stock | IPO
Public Offering
Shares sold				8,941,250
Public offering price per share				9.00
Net proceeds from sale of shares in IPO				69,200,000
Common stock | Over-allotment option
Public Offering
Shares sold				1,166,250
Common stock | Follow-on Public Offering
Public Offering
Shares sold in public follow-on offering (in shares)					1,960,390
Sale of stock by selling shareholders (in shares)					5,402,601
Public offering price per share					$ 17.00
Net proceeds from sale of stock					$ 30,800,000
Common stock | Series A preferred stock
Reverse Stock Split
Ratio for reverse stock split	66.67