STATEMENTS OF CHANGES IN COMMON EQUITY AND COMPREHENSIVE INCOME (Parenthetical) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Change In Accumulated Other Comprehensive Loss Balances [Roll Forward]
Beginning balance	$ (10,869)	[1]	$ (13,285)	[1]
Changes During Year	(3,610)	[2]	2,416	[2]	(1,156)	[2]
Ending balance	(14,479)	[1]	(10,869)	[1]	(13,285)	[1]
Postretirement Liability Adjustment [Member]
Change In Accumulated Other Comprehensive Loss Balances [Roll Forward]
Beginning balance	(10,672)	[1],[2]	(12,958)	[1],[2]	(11,275)	[2]
Changes During Year	(3,165)	[2]	2,286	[2]	(1,683)	[2]
Ending balance	(13,837)	[1],[2]	(10,672)	[1],[2]	(12,958)	[1],[2]
Unrealized Gain (Loss) on Available-for-Sale Securities [Member]
Change In Accumulated Other Comprehensive Loss Balances [Roll Forward]
Beginning balance	397	[1],[2]	294	[1],[2]	(206)	[2]
Changes During Year	(472)	[2]	103	[2]	500	[2]
Ending balance	(75)	[1],[2]	397	[1],[2]	294	[1],[2]
Unrealized Gain (Loss) on Derivatives [Member]
Change In Accumulated Other Comprehensive Loss Balances [Roll Forward]
Beginning balance	(594)	[1],[2]	(621)	[1],[2]	(648)	[2]
Changes During Year	27	[2]	27	[2]	27	[2]
Ending balance	(567)	[1],[2]	(594)	[1],[2]	(621)	[1],[2]
Accumulated Other Comprehensive Income (Loss) [Member]
Change In Accumulated Other Comprehensive Loss Balances [Roll Forward]
Beginning balance	(10,869)	[2]	(13,285)	[2]	(12,129)	[2]
Changes During Year	(3,610)	[2]	2,416	[2]	(1,156)	[2]
Ending balance	$ (14,479)	[2]	$ (10,869)	[2]	$ (13,285)	[2]

[1] Determined using a combined statutory tax rate of 40% and 41% in 2014 and 2013, respectively.

[2] Determined using a combined statutory tax rate of 40% in 2014 and 41% in 2013 and 2012.