Earnings Per Share - Calculations of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Calculation of Basic Earnings Per Share:
Net Income	$ 9,857	$ 3,219	$ 5,060	$ 10,727	$ 7,957	$ 2,397	$ 3,520	$ 13,747	$ 28,863	$ 27,622	$ 26,056
Weighted shares outstanding - Basic									9,586,144	9,555,799	9,474,554
Basic Earnings Per Share	$ 1.03	$ 0.34	$ 0.53	$ 1.12	$ 0.83	$ 0.25	$ 0.37	$ 1.44	$ 3.01	$ 2.89	$ 2.75
Reconciliation of Numerator:
Net Income	9,857	3,219	5,060	10,727	7,957	2,397	3,520	13,747	28,863	27,622	26,056
Effect of 8.25% Convertible debentures									53	61	73
Adjusted numerator - Diluted									$ 28,916	$ 27,683	$ 26,129
Reconciliation of Denominator:
Weighted shares outstanding - Basic									9,586,144	9,555,799	9,474,554
Effect of dilutive securities:
Share-based Compensation									23,499	23,792	22,550
8.25% Convertible debentures									61,864	71,467	85,270
Adjusted denominator - Diluted									9,671,507	9,651,058	9,582,374
Diluted Earnings Per Share	$ 1.02	$ 0.33	$ 0.52	$ 1.11	$ 0.83	$ 0.25	$ 0.37	$ 1.43	$ 2.99	$ 2.87	$ 2.73