STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net Income		$ 66,578	$ 66,483	$ 62,236
Other Comprehensive Income (Loss), Net of Tax:
Postretirement Liability Adjustment	[1]	1,617	(3,165)	2,286
Unrealized (Loss) Gain on Available-for-Sale Securities	[1]	(23)	(472)	103
Unrealized Gain on Derivatives - Other	[1]	23	27	27
Other Comprehensive Income (Loss) - Net of Tax	[1],[2]	1,617	(3,610)	2,416
Comprehensive Income		$ 68,195	$ 62,873	$ 64,652
Additional Statement Information [Abstract]
Combined statutory rate		40.00%	40.00%	41.00%

[1] Determined using a combined statutory tax rate of 40% in each of 2015 and 2014 and 41% in 2013.

[2] Determined using a combined statutory tax rate of 40% in each of 2015 and 2014 and 41% in 2013.