Calculation of Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Calculation of Basic Earnings Per Share:
Net Income	$ 14,869	$ 10,727	$ 28,863
Weighted shares outstanding - Basic	9,601,529	9,571,270
Basic Earnings Per Share	$ 1.55	$ 1.12
Reconciliation of Numerator:
Net Income	14,869	10,727	28,863
Effect of 8.25% Convertible debentures	11	14
Adjusted numerator-Diluted	$ 14,880	$ 10,741
Reconciliation of Denominator:
Weighted shares outstanding-Basic	9,601,529	9,571,270
Effect of dilutive securities:
Share-based Compensation	23,132	29,931
8.25% Convertible debentures	54,289	65,684
Adjusted denominator-Diluted	9,678,950	9,666,885
Diluted Earnings Per Share	$ 1.54	$ 1.11