Statements of Consolidated Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net Income	$ 97,046		$ 81,593		$ 91,608
Other Comprehensive (Loss) Income, Net of Tax:
Postretirement Liability Adjustment	(9,160)	[1]	4,934	[1]	(1,842)	[1]
Unrealized (Loss) Gain on Available-for-Sale Securities	(472)	[1]	103	[1]	500	[1]
Unrealized Gain (Loss) on Derivatives - Other	247	[1]	265	[1]	(92)	[1]
Other Comprehensive (Loss) Income of Affiliated Companies	(113)	[1]	5,043	[1]	(1,233)	[1]
Other Comprehensive (Loss) Income - Net of Tax	(9,498)	[1]	10,345	[1]	(2,667)	[1]
Comprehensive Income	$ 87,548		$ 91,938		$ 88,941
Additional Statement Information [Abstract]
Combined statutory tax rate	40.00%		41.00%		41.00%

[1] determined using a combined statutory tax rate of 41%.