Statements of Common Stockholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Retained Earnings [Member]	Total
Balance at Dec. 31, 2013	$ 80,545	$ 22,966	$ 103,511
Balance (in shares) at Dec. 31, 2013	12,979,281
Net income	$ 0	11,484	11,484
Dividends	0	(7,443)	(7,443)
Retirement of common stock	$ (5,692)	0	$ (5,692)
Retirement of common stock (in shares)	(282,570)		(282,570)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 2,703	0	$ 2,703
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	133,810
Balance at Dec. 31, 2014	$ 77,556	27,007	104,563
Balance (in shares) at Dec. 31, 2014	12,830,521
Net income	$ 0	12,489	12,489
Dividends	0	(7,743)	(7,743)
Retirement of common stock	$ (2,546)	0	$ (2,546)
Retirement of common stock (in shares)	(121,012)		(121,012)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 2,307	0	$ 2,307
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	102,868
Balance at Dec. 31, 2015	$ 77,317	31,753	$ 109,070
Balance (in shares) at Dec. 31, 2015	12,812,377		12,812,377
Net income	$ 0	11,846	$ 11,846
Dividends	0	(8,051)	(8,051)
Retirement of common stock	$ (1,339)	0	$ (1,339)
Retirement of common stock (in shares)	(46,771)		(46,771)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 2,513	0	$ 2,513
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	85,458
Stock-based compensation	$ 22	0	22
Stock-based compensation (in shares)	1,231
Balance at Dec. 31, 2016	$ 78,513	$ 35,548	$ 114,061
Balance (in shares) at Dec. 31, 2016	12,852,295		12,852,295