Nature of the Business (Details) (USD $) In Millions, except Share data, unless otherwise specified	9 Months Ended	1 Months Ended
	Sep. 30, 2012 item	Apr. 30, 2012 Initial public offering Common stock
Nature of the Business
Number of targeted therapeutic oncology candidates	5
Unrestricted cash and cash equivalents and available-for-sale securities	$ 86.7
Public Offering
Aggregate shares of common stock sold in connection with initial public offering		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