Dividends Paid - Additional Information (Detail) (USD $)	1 Months Ended		3 Months Ended		9 Months Ended
	Sep. 17, 2013	Jul. 16, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Dividends Payable [Line Items]
Dividends declared per common share	$ 0.30	$ 0.30	$ 0.60	$ 0.54	$ 1.14	$ 1.02
Cash dividend, payment date	Nov. 15, 2013	Aug. 15, 2013
Cash dividend, record date	Nov. 01, 2013	Aug. 01, 2013
Cash dividend, declaration date	Sep. 17, 2013	Jul. 16, 2013