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
	Mar. 31, 2013	Mar. 31, 2012
Numerator for basic and diluted earnings per share:
Net income	$ 17,577	$ 30,291
Effect of reallocating undistributed earnings of participating securities	(117)	(165)
Net income available under the two-class method	$ 17,460	$ 30,126
Denominator:
Denominator for basic earnings per share- weighted-average shares	22,990,048	22,973,986
Effect of dilutive securities:
Employee stock options		1,280
Dilutive potential common shares		1,280
Denominator for diluted earnings per share- adjusted weighted-average shares	22,990,048	22,975,266
Basic earnings per share	$ 0.76	$ 1.31
Diluted earnings per share	$ 0.76	$ 1.31