Statements of Common Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Common Stock [Member]	Retained Earnings [Member]	Total
Balance at Dec. 31, 2017	$ 79,201	$ 40,204	$ 119,405
Balance (in shares) at Dec. 31, 2017	12,872,742
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 0	9,699	9,699
Cash dividends declared	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
Balance at Jun. 30, 2018	$ 80,404	41,807	122,211
Balance (in shares) at Jun. 30, 2018	12,914,267
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 0	3,800	3,800
Cash dividends declared	0	(2,152)	(2,152)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 402	0	402
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	13,818
Stock-based compensation	$ 11	0	11
Stock-based compensation (in shares)	0
Balance at Sep. 30, 2018	$ 80,817	43,455	124,272
Balance (in shares) at Sep. 30, 2018	12,928,085
Balance at Dec. 31, 2018	$ 81,305	44,890	$ 126,195
Balance (in shares) at Dec. 31, 2018	12,943,536		12,943,536
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 0	11,013	$ 11,013
Cash dividends declared	0	(6,739)	(6,739)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 1,220	0	1,220
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	34,327
Stock-based compensation	$ 119	0	119
Stock-based compensation (in shares)	6,963
Balance at Sep. 30, 2019	$ 82,644	49,164	$ 131,808
Balance (in shares) at Sep. 30, 2019	12,984,826		12,984,826
Balance at Jun. 30, 2019	$ 82,183	46,931	$ 129,114
Balance (in shares) at Jun. 30, 2019	12,974,287
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 0	4,483	4,483
Cash dividends declared	0	(2,250)	(2,250)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 421	0	421
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	10,539
Stock-based compensation	$ 40	0	40
Stock-based compensation (in shares)	0
Balance at Sep. 30, 2019	$ 82,644	$ 49,164	$ 131,808
Balance (in shares) at Sep. 30, 2019	12,984,826		12,984,826