Dividends Paid - Additional Information (Detail) (USD $)	1 Months Ended		3 Months Ended
	Mar. 19, 2013	Jan. 22, 2013	Mar. 31, 2013	Mar. 31, 2012
Dividends Payable [Line Items]
Dividends declared per common share	$ 0.27	$ 0.27	$ 0.54	$ 0.48
Cash dividend, payment date	May 15, 2013	Feb. 15, 2013
Cash dividend, record date	May 01, 2013	Feb. 01, 2013
Cash dividend, declaration date	Mar. 19, 2013	Jan. 22, 2013