Statements of Common Stockholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Retained Earnings [Member]	Total
Balance at Dec. 31, 2014	$ 77,556	$ 27,007	$ 104,563
Balance (in shares) at Dec. 31, 2014	12,830,521
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 0	12,489	12,489
Dividends	0	(7,743)	(7,743)
Retirement of common stock	$ (2,546)	0	$ (2,546)
Retirement of common stock (in shares)	(121,012)		(121,012)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 2,307	0	$ 2,307
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	102,868
Balance at Dec. 31, 2015	$ 77,317	31,753	109,070
Balance (in shares) at Dec. 31, 2015	12,812,377
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 0	11,846	11,846
Dividends	0	(8,051)	(8,051)
Retirement of common stock	$ (1,339)	0	$ (1,339)
Retirement of common stock (in shares)	(46,771)		(46,771)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 2,513	0	$ 2,513
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	85,458
Stock-based compensation	$ 22	0	22
Stock-based compensation (in shares)	1,231
Balance at Dec. 31, 2016	$ 78,513	35,548	$ 114,061
Balance (in shares) at Dec. 31, 2016	12,852,295		12,852,295
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 0	12,974	$ 12,974
Dividends	0	(8,318)	(8,318)
Retirement of common stock	$ (1,263)	0	$ (1,263)
Retirement of common stock (in shares)	(37,229)		(37,229)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 1,905	0	$ 1,905
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	56,171
Stock-based compensation	$ 46	0	46
Stock-based compensation (in shares)	1,505
Balance at Dec. 31, 2017	$ 79,201	$ 40,204	$ 119,405
Balance (in shares) at Dec. 31, 2017	12,872,742		12,872,742