Statements of Common Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Common Stock [Member]	Retained Earnings [Member]	Total
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	8,904	8,904
Dividends	0	(5,994)	(5,994)
Retirement of common stock	$ (474)	0	$ (474)
Retirement of common stock (in shares)	(16,849)		(16,849)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 2,113	0	$ 2,113
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	74,168
Balance at Sep. 30, 2016	$ 78,956	34,663	113,619
Balance (in shares) at Sep. 30, 2016	12,869,696
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	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)		(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		12,859,432