Earnings Per Share - Calculations of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Calculation of Basic Earnings Per Share:
Net Income		$ 8,619	$ 5,119	$ 6,294	$ 21,109	$ 10,097	$ 3,180	$ 5,134	$ 17,681	$ 41,140	$ 36,092	$ 32,787
Weighted shares outstanding - Basic (in shares)										15,094,423	14,551,308	14,430,962
Basic Earnings Per Share (in usd per share)		$ 0.56	$ 0.34	$ 0.41	$ 1.45	$ 0.69	$ 0.22	$ 0.35	$ 1.22	$ 2.73	$ 2.48	$ 2.27
Reconciliation of Numerator:
Net Income		$ 8,619	$ 5,119	$ 6,294	$ 21,109	$ 10,097	$ 3,180	$ 5,134	$ 17,681	$ 41,140	$ 36,092	$ 32,787
Effect of 8.25% Convertible debentures										0	0	43
Adjusted numerator — Diluted										$ 41,140	$ 36,092	$ 32,830
Reconciliation of Denominator:
Weighted shares outstanding - Basic (in shares)										15,094,423	14,551,308	14,430,962
Effect of dilutive securities:
Share-based Compensation										48,950	53,636	37,866
8.25% Convertible debentures	[1]									0	0	74,618
Adjusted denominator — Diluted										15,143,373	14,604,944	14,543,446
Diluted Earnings Per Share (in usd per share)		$ 0.56	$ 0.33	$ 0.41	$ 1.44	$ 0.69	$ 0.22	$ 0.35	$ 1.21	$ 2.72	$ 2.47	$ 2.26

[1] (1) As of March 1, 2014, we no longer have any outstanding convertible debentures.