Computation of Basic and Diluted Earnings per Share under Two-Class Method (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	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
Numerator for basic and diluted earnings per share:
Net income	$ 19,827	$ 23,898	$ 31,917	$ 30,291	$ 33,117	$ 36,911	$ 29,944	$ 30,164	$ 105,933	$ 130,136	$ 78,716
Effect of reallocating undistributed earnings of participating securities									(553)	(749)	(485)
Net income available under the two-class method									$ 105,380	$ 129,387	$ 78,231
Denominator:
Denominator for basic earnings per share-weighted-average shares									22,968,696	23,011,087	22,969,360
Effect of dilutive securities:
Dilutive potential common shares									625	1,332	7,836
Denominator for diluted earnings per share-adjusted weighted-average shares									22,969,321	23,012,419	22,977,196
Basic earnings per share	$ 0.86	$ 1.04	$ 1.38	$ 1.31	$ 1.43	$ 1.59	$ 1.29	$ 1.30	$ 4.59	$ 5.62	$ 3.41
Diluted earnings per share	$ 0.86	$ 1.04	$ 1.38	$ 1.31	$ 1.43	$ 1.59	$ 1.29	$ 1.30	$ 4.59	$ 5.62	$ 3.40
Employee Stock Options
Effect of dilutive securities:
Share based payment arrangements									625	1,332	3,802
Deferred Stock Awards
Effect of dilutive securities:
Share based payment arrangements											4,034