Quarterly Data - Unaudited (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Operating Revenues	$ 34,626	$ 39,920	$ 35,277	$ 31,769	$ 32,738	$ 37,769	$ 33,393	$ 30,698	$ 141,592	$ 134,598	$ 138,077
Operating Income	8,331	13,177	9,385	6,527	7,559	11,983	8,950	7,028	37,420	35,520	37,142
Net Income	$ 8,307	$ 12,737	$ 9,713	$ 7,668	$ 8,071	$ 11,119	$ 8,146	$ 6,552	$ 38,425	$ 33,888	$ 32,452
Basic Earnings per Share	$ 0.47	$ 0.73	$ 0.55	$ 0.44	$ 0.46	$ 0.67	$ 0.49	$ 0.40	$ 2.19	$ 2.02	$ 1.97
Diluted Earnings per Share	0.47	0.72	0.55	0.44	0.46	0.67	0.49	0.39	2.18	2.01	1.96
Common divident per share	0.2725	0.2563	0.2563	0.2563	0.2563	0.2400	0.2400	0.2400	$ 1.041	$ 0.976	$ 0.911
High common stock price	76.08	69.89	72.41	69.92	67.69	66.10	63.68	60.48
Low common stock price	$ 61.81	$ 59.61	$ 53.70	$ 48.79	$ 58.75	$ 55.30	$ 52.51	$ 51.02