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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Calculation of Basic Earnings Per Share:
Net Income	$ 10,097	$ 3,180	$ 5,134	$ 17,681	$ 9,683	$ 3,879	$ 4,356	$ 14,869	$ 36,092	$ 32,787	$ 28,863
Weighted shares outstanding - Basic (in shares)									14,551,308	14,430,962	14,379,216
Basic Earnings Per Share (in usd per share)	$ 0.69	$ 0.22	$ 0.35	$ 1.22	$ 0.67	$ 0.27	$ 0.30	$ 1.03	$ 2.48	$ 2.27	$ 2.01
Reconciliation of Numerator:
Net Income	10,097	3,180	5,134	17,681	9,683	3,879	4,356	14,869	36,092	32,787	28,863
Effect of 8.25% Convertible debentures									0	43	53
Adjusted numerator — Diluted									$ 36,092	$ 32,830	$ 28,916
Reconciliation of Denominator:
Weighted shares outstanding - Basic (in shares)									14,551,308	14,430,962	14,379,216
Effect of dilutive securities:
Share-based Compensation									53,636	37,866	35,249
8.25% Convertible debentures									0	74,618	92,796
Adjusted denominator — Diluted									14,604,944	14,543,446	14,507,261
Diluted Earnings Per Share (in usd per share)	$ 0.69	$ 0.22	$ 0.35	$ 1.21	$ 0.67	$ 0.27	$ 0.30	$ 1.03	$ 2.47	$ 2.26	$ 1.99