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		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Calculation of Basic Earnings Per Share:
Net Income	$ 3,879	$ 3,219	$ 23,104	$ 19,006	$ 28,863
Weighted shares outstanding - Basic (in shares)	9,625,435	9,592,417	9,616,269	9,583,316
Basic Earnings Per Share (in usd per share)	$ 0.40	$ 0.34	$ 2.40	$ 1.98
Reconciliation of Numerator:
Net Income	3,879	3,219	23,104	19,006	28,863
Effect of 8.25% Convertible debentures	11	13	33	41
Adjusted numerator-Diluted	$ 3,890	$ 3,232	$ 23,137	$ 19,047
Reconciliation of Denominator:
Weighted shares outstanding - Basic (in shares)	9,625,435	9,592,417	9,616,269	9,583,316
Effect of dilutive securities:
Share-based Compensation (in shares)	26,123	23,770	23,888	22,684
8.25% Convertible debentures (in shares)	50,776	60,471	52,154	67,681
Adjusted denominator-Diluted (in shares)	9,702,334	9,676,658	9,692,311	9,673,681
Diluted Earnings Per Share (in usd per share)	$ 0.40	$ 0.33	$ 2.39	$ 1.97