Reconciliation of Income and Share Data Used in Basic and Diluted Earnings per Share Computations (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings Per Share Disclosure [Line Items]
Net earnings	$ 37,700,000	$ 19,200,000	$ 15,100,000	$ 71,300,000	$ 37,400,000	$ 36,600,000	$ 66,300,000	$ 58,200,000	$ 143,251,000	$ 198,502,000	$ 270,999,000
Preferred dividends											6,158,000
Income available to common stockholders for basic earnings per share									143,251,000	198,502,000	264,841,000
Preferred dividends											6,158,000
Effect of other dilutive securities									(500,000)		400,000
Income available to common stockholders for diluted earnings per share									$ 142,800,000	$ 198,500,000	$ 271,400,000
Weighted average shares outstanding applicable to basic earnings per share									8,807,487	9,081,535	9,055,920
Preferred Stock											447,497
Effect of other dilutive securities									4,157		15,060
Adjusted weighted average shares outstanding applicable to diluted earnings per share									8,811,644	9,081,535	9,518,477