Statements of Common Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Common Stock [Member]	Retained Earnings [Member]	Total
Balance at Dec. 31, 2016	$ 78,513	$ 35,548	$ 114,061
Balance (in shares) at Dec. 31, 2016	12,852,295
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 0	5,516	5,516
Dividends	0	(4,116)	(4,116)
Retirement of common stock	$ (1,263)	0	(1,263)
Retirement of common stock (in shares)	(37,229)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 1,018	0	1,018
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	29,418
Stock-based compensation	$ 34	0	34
Stock-based compensation (in shares)	1,505
Balance at Jun. 30, 2017	$ 78,302	36,948	115,250
Balance (in shares) at Jun. 30, 2017	12,845,989
Balance at Dec. 31, 2017	$ 79,201	40,204	$ 119,405
Balance (in shares) at Dec. 31, 2017	12,872,742		12,872,742
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 0	5,899	$ 5,899
Dividends	0	(4,296)	(4,296)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 1,146	0	1,146
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	37,782
Stock-based compensation	$ 57	0	57
Stock-based compensation (in shares)	3,743
Balance at Jun. 30, 2018	$ 80,404	$ 41,807	$ 122,211
Balance (in shares) at Jun. 30, 2018	12,914,267		12,914,267