Dividends Paid - Additional Information (Detail) (USD $)	1 Months Ended	6 Months Ended		1 Months Ended
	Mar. 20, 2012	Jun. 30, 2012	Jun. 30, 2011	Jul. 17, 2012 Subsequent Event
Dividends Payable [Line Items]
Dividends declared per common share	$ 0.24	$ 0.48	$ 0.40	$ 0.27
Cash dividend, record date	May 01, 2012			Aug. 01, 2012
Cash dividend, paid date	May 15, 2012			Aug. 15, 2012
Cash dividend, declaration date	Mar. 20, 2012			Jul. 17, 2012