Quarterly Data - Unaudited (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Operating Revenues	$ 32,738	$ 37,769	$ 33,393	$ 30,698	$ 33,268	$ 38,713	$ 34,919	$ 31,177	$ 134,598	$ 138,077	$ 130,775
Operating Income	7,559	11,983	8,950	7,028	7,153	12,918	10,721	6,350	35,520	37,142	37,798
Net Income	$ 8,071	$ 11,119	$ 8,146	$ 6,552	$ 6,993	$ 12,290	$ 8,675	$ 4,494	$ 33,888	$ 32,452	$ 22,809
Basic Earnings per Share	$ 0.46	$ 0.67	$ 0.49	$ 0.40	$ 0.43	$ 0.75	$ 0.52	$ 0.27	$ 2.02	$ 1.97	$ 1.39
Diluted Earnings per Share	0.46	0.67	0.49	0.39	0.43	0.74	0.52	0.27	2.01	$ 1.96	$ 1.38
Common divident per share	0.2563	0.2400	0.2400	0.2400	0.2400	0.2238	0.2238	0.2238	$ 0.976
High common stock price	67.69	66.10	63.68	60.48	60.31	49.00	45.24	41.45
Low common stock price	$ 58.75	$ 55.30	$ 52.51	$ 51.02	$ 43.12	$ 41.77	$ 34.74	$ 33.96