Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2013		Sep. 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	(23,228)			(23,228)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	43,187		49,369	127,806		145,734
Operating and maintenance expenses	192,073		180,994	553,266		529,476
Total, pre-tax	(259,502)		(274,342)	(591,721)		(570,744)
Tax expense	93,236		81,899	211,551		189,609
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(23,120)			(22,871)
Other comprehensive gain (loss) before reclassifications	11			(2)
Gains reclassified from net accumulated other comprehensive loss	(119)			(355)
Net current period other comprehensive income (loss)	(108)			(357)
Accumulated other comprehensive loss at end of period	(23,228)			(23,228)
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	(195)			(576)
Tax expense	76			221
Total, net of tax	(119)			(355)
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	(184)	[1]		(546)	[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	$ (11)	[2]		$ (30)	[2]

[1] Included in interest charges.

[2] Included in O&M expenses