Dividends Paid - Additional Information (Detail) (USD $)	1 Months Ended		3 Months Ended		9 Months Ended
	Sep. 18, 2012	Jul. 17, 2012	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Dividends Payable [Line Items]
Dividends declared per common share	$ 0.27	$ 0.27	$ 0.54	$ 0.48	$ 1.02	$ 0.88
Cash dividend, record date	Nov. 01, 2012	Aug. 01, 2012
Cash dividend, paid date	Nov. 15, 2012	Aug. 15, 2012
Cash dividend, declaration date	Sep. 18, 2012	Jul. 17, 2012