Dividends - Additional Information (Detail) (USD $)	3 Months Ended			6 Months Ended		1 Months Ended
	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Aug. 09, 2012 Subsequent Event Dividend Declared
Dividends Payable [Line Items]
Cash dividend paid, per share amount	$ 0.12	$ 0.12	$ 0.11	$ 0.24	$ 0.22
Dividends paid on common stock				$ 16,947,000	$ 15,600,000
Cash dividend declared per share amount						$ 0.12
Cash dividend payable date						Sep. 18, 2012
Dividend payable, date of record for common stock holders						Sep. 04, 2012
Expected payment of quarterly dividend						$ 8,400,000