Nature of the Business (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended	1 Months Ended
	Dec. 31, 2012 item	Apr. 30, 2012 Initial public offering Common stock
Nature of the Business
Number of targeted therapeutic oncology candidates	6
Public Offering
Aggregate shares of common stock sold in connection with initial public offering (in shares)		15,042,459
Public offering price (in dollars per share)		$ 7.00
Common stock issued to underwriters under an over-allotment option (in shares)		742,459
Net proceeds from initial public offering		$ 98.1