Statements of Consolidated Comprehensive Income - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net Income		$ 105,107	$ 97,046	$ 81,593
Other Comprehensive (Loss) Income, Net of Tax:
Postretirement Liability Adjustment	[1]	5,518	(9,160)	4,934
Unrealized (Loss) Gain on Available-for-Sale Securities	[1]	(53)	(472)	103
Unrealized Gain on Derivatives - Other	[1]	321	247	265
Other Comprehensive (Loss) Income of Affiliated Companies	[1]	(27)	(113)	5,043
Other Comprehensive (Loss) Income - Net of Tax	[1]	5,759	(9,498)	10,345
Comprehensive Income		$ 110,866	$ 87,548	$ 91,938
Additional Statement Information [Abstract]
Combined statutory tax rate		40.00%	40.00%	41.00%

[1] For 2015 and 2014, determined using a combined average statutory tax rate of 40%. For 2013, determined using a combined statutory tax rate of 41%.