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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Numerator for basic and diluted earnings per share:
Net income	$ 16,307	$ 31,917	$ 33,883	$ 62,208
Effect of reallocating undistributed earnings of participating securities	(107)	(171)	(224)	(336)
Net income available under the two-class method	$ 16,200	$ 31,746	$ 33,659	$ 61,872
Denominator:
Denominator for basic earnings per share- weighted-average shares	22,953,453	22,961,130	22,971,649	22,967,558
Effect of dilutive securities:
Employee stock options		1,220		1,250
Dilutive potential common shares		1,220		1,250
Denominator for diluted earnings per share- adjusted weighted-average shares	22,953,453	22,962,350	22,971,649	22,968,808
Basic earnings per share	$ 0.71	$ 1.38	$ 1.47	$ 2.69
Diluted earnings per share	$ 0.71	$ 1.38	$ 1.47	$ 2.69