Subsequent Event (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended
	Aug. 18, 2014	Apr. 06, 2015	Mar. 31, 2015	Dec. 31, 2014
Subsequent Events
Number of shares issued under underwritten public offering	10,235,000
Number of share's exercise in full of over-allotment option by underwriter	1,335,000
Shares price (in dollars per share)	$ 11.25
Underwriter's discount and offering expenses	$ 7.1
Net proceeds from the sale of shares	108.0
Common stock, issued shares			50,001,770	49,806,131
Common stock, outstanding shares			50,001,770	49,806,131
Subsequent event
Subsequent Events
Number of shares issued under underwritten public offering		11,500,000
Number of share's exercise in full of over-allotment option by underwriter		1,500,000
Shares price (in dollars per share)		$ 20.65
Underwriter's discount and offering expenses		14.5
Net proceeds from the sale of shares		$ 223.0
Common stock, issued shares		61,501,770
Common stock, outstanding shares		61,501,770