Statements of Common Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Common Stock [Member]	Retained Earnings [Member]	Total
Balance at Dec. 31, 2014	$ 77,556	$ 27,007	$ 104,563
Balance (in shares) at Dec. 31, 2014	12,830,521
Net income	$ 0	8,973	8,973
Dividends	0	(5,753)	(5,753)
Retirement of common stock	$ (2,495)	0	$ (2,495)
Retirement of common stock (in shares)	(118,812)		(118,812)
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans	$ 1,762	0	$ 1,762
Issuance of common stock under dividend reinvestment, direct stock and employee stock purchase plans (in shares)	80,349
Balance at Sep. 30, 2015	$ 76,823	30,227	107,050
Balance (in shares) at Sep. 30, 2015	12,792,058
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	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		12,869,696