Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period					$ (23,338)
Accumulated other comprehensive loss at end of period	(23,573)				(23,573)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	188,901		188,152		364,425		361,193
Total, pre-tax	(139,684)		(151,657)		(320,584)		(332,219)
Tax expense	49,892		54,358		112,389		118,315
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(23,461)		(22,982)		(23,338)		(22,871)
Other comprehensive gain (loss) before reclassifications	5		(20)		2		(13)
Gains reclassified from net accumulated other comprehensive loss	(117)		(118)		(237)		(236)
Net current period other comprehensive loss	(112)		(138)		(235)		(249)
Accumulated other comprehensive loss at end of period	(23,573)		(23,120)		(23,573)		(23,120)
Gains and Losses on Cash Flow Hedges | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	(189)		(189)		(382)		(381)
Tax expense	72		71		145		145
Total, net of tax	(117)		(118)		(237)		(236)
Gains and Losses on Cash Flow Hedges | Interest Rate Derivatives | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	(182)	[1]	(182)	[1]	(362)	[1]	(362)	[1]
Gains and Losses on Cash Flow Hedges | Vehicle Fuel Derivatives | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	$ (7)	[2]	$ (7)	[2]	$ (20)	[2]	$ (19)	[2]

[1] Included in interest charges.

[2] Included in O&M expenses