CONDENSED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Statement of Other Comprehensive Income [Abstract]
Net Income	$ 58,241		$ 52,889		$ 43,925
Other Comprehensive Gain (Loss) - Net of Tax:
Postretirement Liability Adjustment	(1,683)	[1]	(2,676)	[1]	(3,156)	[1]
Unrealized Gain (Loss) on Available-for-Sale Securities	500	[1]	(360)	[1]	339	[1]
Unrealized Gain on Derivatives - Other	27	[1]	27	[1]	27	[1]
Other Comprehensive Income (Loss), Net of Tax	(1,156)	[1]	(3,009)	[1]	(2,790)	[1]
Comprehensive Income (Loss)	$ 57,085		$ 49,880		$ 41,135
Combined statutory rate	41.00%

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