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		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Numerator for basic and diluted earnings per share:
Net income	$ 23,898	$ 36,911	$ 86,106	$ 97,019
Effect of reallocating undistributed earnings of participating securities	(122)	(212)	(458)	(562)
Net income available under the two-class method	$ 23,776	$ 36,699	$ 85,648	$ 96,457
Denominator:
Denominator for basic earnings per share-weighted-average shares	22,963,318	23,026,741	22,966,134	23,022,836
Effect of dilutive securities:
Dilutive potential common shares		1,351	833	1,333
Denominator for diluted earnings per share-adjusted weighted-average shares	22,963,318	23,028,092	22,966,967	23,024,169
Basic earnings per share	$ 1.04	$ 1.59	$ 3.73	$ 4.19
Diluted earnings per share	$ 1.04	$ 1.59	$ 3.73	$ 4.19
Employee Stock Options
Effect of dilutive securities:
Employee stock options		1,351	833	1,333