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, 2014	Mar. 31, 2013	Dec. 31, 2013
Calculation of Basic Earnings Per Share:
Net Income	$ 17,681	$ 14,869	$ 32,787
Weighted shares outstanding - Basic (in shares)	9,658,431	9,601,529
Basic Earnings Per Share (in usd per share)	$ 1.83	$ 1.55
Reconciliation of Numerator:
Net Income	17,681	14,869	32,787
Effect of 8.25% Convertible debentures	0	11
Adjusted numerator-Diluted	$ 17,681	$ 14,880
Reconciliation of Denominator:
Weighted shares outstanding - Basic (in shares)	9,658,431	9,601,529
Effect of dilutive securities:
Share-based Compensation (in shares)	35,003	23,132
8.25% Convertible debentures (in shares)	0	54,289
Adjusted denominator-Diluted (in shares)	9,693,434	9,678,950
Diluted Earnings Per Share (in usd per share)	$ 1.82	$ 1.54