Aggregate dividends paid during the period for each share of common stock outstanding.

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Dividends - Additional Information (Detail) (USD $)	3 Months Ended				9 Months Ended		1 Months Ended
Dividends - Additional Information (Detail) (USD $)	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Nov. 08, 2012 Subsequent Event Dividend Declared
Dividends Payable [Line Items]
Cash dividend paid, per share amount	$ 0.12	$ 0.12	$ 0.12	$ 0.11	$ 0.36	$ 0.33
Dividends paid on common stock					$ 25,371,000	$ 23,361,000
Cash dividend declared per share amount							$ 0.12
Cash dividend payable date							Dec. 17, 2012
Dividend payable, date of record for common stock holders							Dec. 03, 2012
Expected payment of quarterly dividend							$ 8,400,000