Reconciliation of Earnings and Share Data used in Basic and Diluted Earnings per Share Computations (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share Disclosure [Line Items]
Net earnings	$ (92,600,000)	$ 125,400,000	$ 109,300,000	$ 560,100,000	$ 37,700,000	$ 19,200,000	$ 15,100,000	$ 71,300,000	$ 702,244,000	$ 143,251,000	$ 198,502,000
Effect of dilutive securities										(500,000)
Income available to common stockholders for diluted earnings per share									$ 702,200,000	$ 142,800,000	$ 198,500,000
Weighted average common shares outstanding applicable to basic earnings per share									15,441,578	8,807,487	9,081,535
Effect of dilutive securities										4,157
Adjusted weighted average common shares outstanding applicable to diluted earnings per share									15,441,578	8,811,644	9,081,535