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	9,447	9,447
Dividends	0	(6,175)	(6,175)
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	$ 1,462	0	1,462
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	42,861
Stock-based compensation	$ 41	0	41
Stock-based compensation (in shares)	1,505
Balance at Sep. 30, 2017	$ 78,753	38,820	117,573
Balance (in shares) at Sep. 30, 2017	12,859,432
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	9,699	$ 9,699
Dividends	0	(6,448)	(6,448)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 1,548	0	1,548
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	51,600
Stock-based compensation	$ 68	0	68
Stock-based compensation (in shares)	3,743
Balance at Sep. 30, 2018	$ 80,817	$ 43,455	$ 124,272
Balance (in shares) at Sep. 30, 2018	12,928,085		12,928,085