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								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator for basic and diluted earnings per share:
Net income	$ 399	$ 13,744	$ 23,017	$ 18,427	$ 20,854	$ 30,148	$ 16,307	$ 17,577	$ 55,588	$ 84,886	$ 105,933
Effect of reallocating undistributed earnings of participating securities									(376)	(530)	(553)
Net income available under the two-class method									$ 55,212	$ 84,356	$ 105,380
Denominator:
Denominator for basic earnings per share-weighted-average shares									22,946,395	22,957,013	22,968,696
Effect of dilutive potential common shares											625
Denominator for diluted earnings per share-adjusted weighted-average shares									22,946,395	22,957,013	22,969,321
Basic earnings per share	$ 0.02	$ 0.60	$ 1.00	$ 0.80	$ 0.90	$ 1.31	$ 0.71	$ 0.76	$ 2.41	$ 3.67	$ 4.59
Diluted earnings per share	$ 0.02	$ 0.60	$ 1.00	$ 0.80	$ 0.90	$ 1.31	$ 0.71	$ 0.76	$ 2.41	$ 3.67	$ 4.59