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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Numerator for basic and diluted earnings per share:
Net income	$ 13,744	$ 30,148	$ 55,189	$ 64,031
Effect of reallocating undistributed earnings of participating securities	(90)	(183)	(379)	(410)
Net income available under the two-class method	$ 13,654	$ 29,965	$ 54,810	$ 63,621
Denominator:
Denominator for basic earnings per share-weighted-average shares	22,944,840	22,940,794	22,946,956	22,961,251
Effect of dilutive potential common shares	0	0	0	0
Denominator for diluted earnings per share-adjusted weighted-average shares	22,944,840	22,940,794	22,946,956	22,961,251
Basic earnings per share	$ 0.60	$ 1.31	$ 2.39	$ 2.77
Diluted earnings per share	$ 0.60	$ 1.31	$ 2.39	$ 2.77