Subsequent Events - Additional Information (Detail) (USD $)	3 Months Ended								12 Months Ended			1 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jan. 23, 2013 Dividend Declared
Subsequent Event [Line Items]
Subsequent event description												The Board of Directors of the Company declared a quarterly dividend
Dividends declared per common share	$ (0.45)	$ (0.45)	$ (0.45)	$ (0.45)	$ (0.45)	$ (0.45)	$ (0.45)	$ (0.45)	$ 1.80	$ 1.80	$ 1.80	$ 0.45
Dividend payable date												Mar. 20, 2013
Dividend record date												Mar. 06, 2013