Dividends - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended		1 Months Ended
	Mar. 26, 2012	Mar. 31, 2012	Mar. 31, 2011	May 10, 2012 Dividend Declared
Dividends Payable [Line Items]
Cash dividend paid, per share amount	$ 0.12	$ 0.12	$ 0.11
Dividends paid on common stock	$ 8,500,000	$ 8,487,000	$ 7,798,000
Cash dividend declared per share amount				$ 0.12
Cash dividend payable date				Jun. 15, 2012
Dividend payable, date of record for common stock holders				Jun. 01, 2012
Expected payment of quarterly dividend				$ 8,500,000