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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Numerator for basic and diluted earnings per share:
Net income	$ 30,148	$ 23,898	$ 64,031	$ 86,106
Effect of reallocating undistributed earnings of participating securities	(183)	(122)	(410)	(458)
Net income available under the two-class method	$ 29,965	$ 23,776	$ 63,621	$ 85,648
Denominator:
Denominator for basic earnings per share-weighted-average shares	22,940,794	22,963,318	22,961,251	22,966,134
Effect of dilutive securities:
Employee stock options				833
Dilutive potential common shares				833
Denominator for diluted earnings per share-adjusted weighted-average shares	22,940,794	22,963,318	22,961,251	22,966,967
Basic earnings per share	$ 1.31	$ 1.04	$ 2.77	$ 3.73
Diluted earnings per share	$ 1.31	$ 1.04	$ 2.77	$ 3.73