Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period				$ (22,871)
Accumulated other comprehensive loss at end of period	(23,120)			(23,120)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	(43,231)		(48,074)	(84,619)		(96,365)
Operating and maintenance expenses	(188,152)		(180,100)	(361,193)		(348,482)
Total, pre-tax	151,657		150,868	332,219		296,402
Tax expense	54,358		55,461	118,315		107,710
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(22,982)			(22,871)
Other comprehensive income (loss) before reclassifications	(20)			(13)
(Gains) losses reclassified from net accumulated other comprehensive loss	(118)			(236)
Net current period other comprehensive income (loss)	(138)			(249)
Accumulated other comprehensive loss at end of period	(23,120)			(23,120)
Gains and Losses on Cash Flow Hedges [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	(189)			(381)
Tax expense	(71)			(145)
Total, net of tax	118			236
Gains and Losses on Cash Flow Hedges [Member] | Interest Rate Derivatives [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	182	[1]		362	[1]
Gains and Losses on Cash Flow Hedges [Member] | Vehicle Fuel And Other Commodity Derivatives [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	$ 7	[2]		$ 19	[2]

[1] Included in interest charges.

[2] Included in O&M expenses