Equity Financing - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	2 Months Ended	12 Months Ended	1 Months Ended
	Apr. 30, 2011	Oct. 31, 2012	Dec. 31, 2012	Jan. 31, 2013 Subsequent Event
Equity Note [Line Items]
Common stock issued	3,800,000	2,000,000	4,900,000	87,000
Common stock average price per share	$ 9.00		$ 25.25
Net proceeds from issue of common stock	$ 32.1	$ 36.2	$ 118.1	$ 2.1