Computation of Basic and Diluted Income Per Share (Detail) (USD $) In Thousands, except Per 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
Schedule of Earnings Per Share, Basic and Diluted, by Common Class
Net Income	$ 5,768	$ 7,673	$ 6,506	$ 6,635	$ 4,894	$ 7,259	$ 6,099	$ 5,377	$ 26,582	$ 23,629	$ 26,038
Weighted Average Basic Shares Outstanding									2,010	2,016	2,019
Add: Effect of dilutive securities									7	11	12
Weighted average diluted shares outstanding									2,017	2,027	2,031
Net Income per share
Basic	$ 2.88	$ 3.82	$ 3.23	$ 3.28	$ 2.42	$ 3.60	$ 3.03	$ 2.67	$ 13.22	$ 11.72	$ 12.90
Diluted	$ 2.87	$ 3.81	$ 3.22	$ 3.28	$ 2.42	$ 3.59	$ 3.02	$ 2.65	$ 13.18	$ 11.66	$ 12.82