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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Numerator for basic and diluted earnings per share:
Net income	$ 20,854	$ 30,148	$ 16,307	$ 17,577	$ 19,827	$ 23,898	$ 31,917	$ 30,291	$ 84,886	$ 105,933	$ 130,136
Effect of reallocating undistributed earnings of participating securities									(530)	(553)	(749)
Net income available under the two-class method									$ 84,356	$ 105,380	$ 129,387
Denominator:
Denominator for basic earnings per share-weighted-average shares									22,957,013	22,968,696	23,011,087
Effect of dilutive securities:
Dilutive potential common shares										625	1,332
Denominator for diluted earnings per share-adjusted weighted-average shares									22,957,013	22,969,321	23,012,419
Basic earnings per share	$ 0.90	$ 1.31	$ 0.71	$ 0.76	$ 0.86	$ 1.04	$ 1.38	$ 1.31	$ 3.67	$ 4.59	$ 5.62
Diluted earnings per share	$ 0.90	$ 1.31	$ 0.71	$ 0.76	$ 0.86	$ 1.04	$ 1.38	$ 1.31	$ 3.67	$ 4.59	$ 5.62
Employee Stock Options
Effect of dilutive securities:
Share based payment arrangements										625	1,332
Deferred Stock Awards
Effect of dilutive securities:
Share based payment arrangements