Dividends Paid - Additional Information (Detail) (USD $)	1 Months Ended	6 Months Ended		1 Months Ended
	Mar. 19, 2013	Jun. 30, 2013	Jun. 30, 2012	Jul. 16, 2013 Subsequent Event
Dividends Payable [Line Items]
Dividends declared per common share	$ 0.27	$ 0.54	$ 0.48	$ 0.30
Cash dividend, payment date	May 15, 2013			Aug. 15, 2013
Cash dividend, record date	May 01, 2013			Aug. 01, 2013
Cash dividend, declaration date	Mar. 19, 2013			Jul. 16, 2013