Statements of Common Stockholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Retained Earnings [Member]	Total
Balance at Dec. 31, 2012	$ 79,299	$ 20,526	$ 99,825
Balance (in shares) at Dec. 31, 2012	12,918,633
Net income	$ 0	9,654	9,654
Dividends	0	(7,214)	(7,214)
Retirement of common stock	$ (1,772)	0	$ (1,772)
Retirement of common stock (in shares)	(94,414)		(94,414)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 3,018	0	$ 3,018
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	155,062
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		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