Statements of Common Stockholders' Equity - 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	13,376	13,376
Cash dividends declared	0	(8,690)	(8,690)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 2,024	0	2,024
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	67,051
Stock-based compensation	$ 80	0	80
Stock-based compensation (in shares)	3,743
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	14,402	$ 14,402
Cash dividends declared	0	(9,083)	(9,083)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 2,505	0	2,505
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	64,399
Stock-based compensation	$ 166	0	166
Stock-based compensation (in shares)	6,963
Balance at Dec. 31, 2019	$ 83,976	$ 50,209	$ 134,185
Balance (in shares) at Dec. 31, 2019	13,014,898		13,014,898