Quarterly Financial Data (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
Quarterly Financial Information [Line Items]
Operating Revenue	$ 31,851	$ 34,044	$ 32,605	$ 33,493	$ 28,497	$ 30,637	$ 30,689	$ 29,239	$ 131,993	$ 119,062	$ 117,704
Operating Income	8,336	10,713	9,495	9,400	7,039	9,677	8,967	7,943	37,944	33,626	38,168
Net Income	$ 5,768	$ 7,673	$ 6,506	$ 6,635	$ 4,894	$ 7,259	$ 6,099	$ 5,377	$ 26,582	$ 23,629	$ 26,038
Income Per Basic Share	$ 2.88	$ 3.82	$ 3.23	$ 3.28	$ 2.42	$ 3.60	$ 3.03	$ 2.67	$ 13.22	$ 11.72	$ 12.90
Income Per Diluted Share	$ 2.87	$ 3.81	$ 3.22	$ 3.28	$ 2.42	$ 3.59	$ 3.02	$ 2.65	$ 13.18	$ 11.66	$ 12.82