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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share Disclosure [Line Items]
Net earnings	$ 113,167,000	$ 125,444,000	$ 423,170,000	$ 794,856,000
Effect of dilutive securities				200,000
Income available to common stockholders for diluted earnings per share	$ 113,200,000	$ 125,400,000	$ 423,200,000	$ 795,100,000
Weighted average common shares outstanding applicable to basic earnings per share	16,766,192	16,931,811	16,792,733	14,943,160
Effect of dilutive securities				19,855
Adjusted weighted average common shares outstanding applicable to diluted earnings per share	16,766,192	16,931,811	16,792,733	14,963,015