Reconciliation Income and Share Data Used in Basic and Diluted Earnings Per Share Computations (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Earnings Per Share Disclosure [Line Items]
Net earnings	$ 15,082,000	$ 66,271,000	$ 86,422,000	$ 124,440,000
Effect of dilutive securities	(100,000)	(100,000)	(200,000)	(500,000)
Income available to common stockholders for diluted earnings per share	$ 15,000,000	$ 66,200,000	$ 86,200,000	$ 123,900,000
Weighted average shares outstanding applicable to basic earnings per share	8,916,620	9,124,187	8,921,950	9,164,443
Effect of dilutive securities	5,352	9,788	5,168	9,489
Adjusted weighted average shares outstanding applicable to diluted earnings per share	8,921,972	9,133,975	8,927,118	9,173,932