Computation of Basic and Diluted Earnings Per Share Under the Two-Class Method (Detail) (USD $) In Thousands, except Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Numerator for basic and diluted earnings per share:
Net income	$ 36,911	$ 20,175	$ 97,019	$ 57,901
Effect of reallocating undistributed earnings of participating securities	(212)	(123)	(562)	(361)
Net income available under the two-class method	$ 36,699	$ 20,052	$ 96,457	$ 57,540
Denominator:
Denominator for basic earnings per share- weighted-average shares	23,026,741	22,966,617	23,022,836	22,968,492
Effect of dilutive securities:
Dilutive potential common shares	1,351	6,975	1,333	8,083
Denominator for diluted earnings per share- adjusted weighted-average shares	23,028,092	22,973,592	23,024,169	22,976,575
Basic earnings per share	$ 1.59	$ 0.87	$ 4.19	$ 2.51
Diluted earnings per share	$ 1.59	$ 0.87	$ 4.19	$ 2.50
Employee Stock Options
Effect of dilutive securities:
Share based payment arrangements (See Note 6)	1,351	2,936	1,333	4,055
Deferred Stock Awards
Effect of dilutive securities:
Share based payment arrangements (See Note 6)		4,039		4,028