Reconciliation of Earnings and Share Data used in Basic and Diluted Earnings per Share Computations (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share Disclosure [Line Items]
Net earnings	$ 125,444,000	$ 19,158,000	$ 794,856,000	$ 105,580,000
Effect of dilutive securities		(1,300,000)	200,000	(1,100,000)
Income available to common stockholders for diluted earnings per share	$ 125,400,000	$ 17,900,000	$ 795,100,000	$ 104,500,000
Weighted average common shares outstanding applicable to basic earnings per share	16,931,811	8,811,807	14,943,160	8,881,601
Effect of dilutive securities		10,749	19,855	9,858
Adjusted weighted average common shares outstanding applicable to diluted earnings per share	16,931,811	8,822,556	14,963,015	8,891,459