Computation of Basic and Diluted Earnings Per Share Under the Two-Class Method (Detail) (USD $) In Thousands, except Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Numerator for basic and diluted earnings per share:
Net income	$ 29,944	$ 18,734	$ 60,108	$ 37,726
Effect of reallocating undistributed earnings of participating securities	(174)	(117)	(350)	(238)
Net income available under the two-class method	$ 29,770	$ 18,617	$ 59,758	$ 37,488
Denominator:
Denominator for basic earnings per share- weighted-average shares	23,027,104	22,971,383	23,020,851	22,969,445
Effect of dilutive securities:
Dilutive potential common shares	1,346	7,555	1,324	8,637
Denominator for diluted earnings per share- adjusted weighted-average shares	23,028,450	22,978,938	23,022,175	22,978,082
Basic earnings per share	$ 1.29	$ 0.81	$ 2.60	$ 1.63
Diluted earnings per share	$ 1.29	$ 0.81	$ 2.60	$ 1.63
Employee Stock Options
Effect of dilutive securities:
Share based payment arrangements (See Note 6)	1,346	3,527	1,324	4,614
Deferred Stock Awards
Effect of dilutive securities:
Share based payment arrangements (See Note 6)		4,028		4,023