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	5,333	5,333
Dividends	0	(3,994)	$ (3,994)
Retirement of common stock (in shares)			0
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 1,568	0	$ 1,568
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	55,359
Balance at Jun. 30, 2016	$ 78,885	33,092	111,977
Balance (in shares) at Jun. 30, 2016	12,867,736
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	5,516	$ 5,516
Dividends	0	(4,116)	(4,116)
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	$ 1,018	0	$ 1,018
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	29,418
Stock-based compensation	$ 34	0	34
Stock-based compensation (in shares)	1,505
Balance at Jun. 30, 2017	$ 78,302	$ 36,948	$ 115,250
Balance (in shares) at Jun. 30, 2017	12,845,989		12,845,989