Computation of Basic and Diluted Earnings Per Share Under the Two-Class Method (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Numerator for basic and diluted earnings per share:
Net income	$ 33,117	$ 36,911	$ 29,944	$ 30,164	$ 20,815	$ 20,175	$ 18,734	$ 18,992	$ 130,136	$ 78,716	$ 52,810
Effect of reallocating undistributed earnings of participating securities									(749)	(485)	(304)
Net income available under the two-class method									$ 129,387	$ 78,231	$ 52,506
Denominator:
Denominator for basic earnings per share-weighted-average shares									23,011,087	22,969,360	23,097,105
Effect of dilutive securities:
Dilutive potential common shares									1,332	7,836	14,587
Denominator for diluted earnings per share-adjusted weighted-average shares									23,012,419	22,977,196	23,111,692
Basic earnings per share	$ 1.43	$ 1.59	$ 1.29	$ 1.30	$ 0.90	$ 0.87	$ 0.81	$ 0.82	$ 5.62	$ 3.41	$ 2.27
Diluted earnings per share	$ 1.43	$ 1.59	$ 1.29	$ 1.30	$ 0.90	$ 0.87	$ 0.81	$ 0.82	$ 5.62	$ 3.40	$ 2.27
Employee Stock Options
Effect of dilutive securities:
Share based payment arrangements (See Note 8)									1,332	3,802	8,723
Deferred Stock Awards
Effect of dilutive securities:
Share based payment arrangements (See Note 8)										4,034	5,864