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, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Calculation of Basic Earnings Per Share:
Net Income	$ 9,683	[1]	$ 3,879	[1]	$ 4,356	[1]	$ 14,869	[1]	$ 9,857	[1]	$ 3,219	[1]	$ 5,060	[1]	$ 10,727	[1]	$ 32,787	$ 28,863	$ 27,622
Weighted shares outstanding - Basic (in shares)																	9,620,641	9,586,144	9,555,799
Basic Earnings Per Share (in usd per share)	$ 1.01	[1]	$ 0.40	[1]	$ 0.45	[1]	$ 1.55	[1]	$ 1.03	[1]	$ 0.34	[1]	$ 0.53	[1]	$ 1.12	[1]	$ 3.41	$ 3.01	$ 2.89
Reconciliation of Numerator:
Net Income	9,683	[1]	3,879	[1]	4,356	[1]	14,869	[1]	9,857	[1]	3,219	[1]	5,060	[1]	10,727	[1]	32,787	28,863	27,622
Effect of 8.25% Convertible debentures																	43	53	61
Adjusted numerator — Diluted																	$ 32,830	$ 28,916	$ 27,683
Reconciliation of Denominator:
Weighted shares outstanding - Basic (in shares)																	9,620,641	9,586,144	9,555,799
Effect of dilutive securities:
Share-based Compensation																	25,244	23,499	23,792
8.25% Convertible debentures																	49,745	61,864	71,467
Adjusted denominator — Diluted																	9,695,630	9,671,507	9,651,058
Diluted Earnings Per Share (in usd per share)	$ 1.00	[1]	$ 0.40	[1]	$ 0.45	[1]	$ 1.54	[1]	$ 1.02	[1]	$ 0.33	[1]	$ 0.52	[1]	$ 1.11	[1]	$ 3.39	$ 2.99	$ 2.87

[1] The sum of the four quarters does not equal the total year due to rounding.