Dividends Paid - Additional Information (Detail) (USD $)	0 Months Ended		3 Months Ended		9 Months Ended
	Sep. 23, 2014	Jul. 22, 2014	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Dividends Payable [Line Items]
Dividends declared per common share	$ 0.33	$ 0.33	$ 0.66	$ 0.60	$ 1.26	$ 1.14
Cash dividend, payment date	Nov. 17, 2014	Aug. 15, 2014
Cash dividend, record date	Nov. 03, 2014	Aug. 01, 2014
Cash dividend, declaration date	Sep. 23, 2014	Jul. 22, 2014