Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss at beginning of period	$ (112,653)
Other comprehensive income (loss) before reclassifications	(23)
(Gains) losses reclassified from net accumulated other comprehensive loss	(944)
Net current period other comprehensive loss	(967)
Accumulated other comprehensive loss at end of period	(113,620)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	139,613		151,830
Operating and maintenance expenses	529,231		510,684
Total, pre-tax	(355,022)		(259,284)
Tax benefit	118,434		75,515
Total, net of tax	(236,588)		(183,769)
Amounts Reclassified from Net Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, net of tax	(944)
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss at beginning of period	(61,241)
Other comprehensive income (loss) before reclassifications	13
(Gains) losses reclassified from net accumulated other comprehensive loss	(305)
Net current period other comprehensive loss	(292)
Accumulated other comprehensive loss at end of period	(61,533)
Gains and Losses on Cash Flow Hedges [Member] | Amounts Reclassified from Net Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	1,124
Tax benefit	(1,429)
Total, net of tax	(305)
Gains and Losses on Cash Flow Hedges [Member] | Interest Rate Derivatives [Member] | Amounts Reclassified from Net Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	1,150	[1]
Gains and Losses on Cash Flow Hedges [Member] | Vehicle Fuel And Other Commodity Derivatives [Member] | Amounts Reclassified from Net Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	(26)	[2]
Unrealized Gains and Losses on Marketable Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss at beginning of period	(99)
Other comprehensive income (loss) before reclassifications	(36)
(Gains) losses reclassified from net accumulated other comprehensive loss	0
Net current period other comprehensive loss	(36)
Accumulated other comprehensive loss at end of period	(135)
Defined Benefit Pension and Postretirement Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss at beginning of period	(51,313)
Other comprehensive income (loss) before reclassifications	0
(Gains) losses reclassified from net accumulated other comprehensive loss	(639)
Net current period other comprehensive loss	(639)
Accumulated other comprehensive loss at end of period	(51,952)
Defined Benefit Pension and Postretirement Items [Member] | Amounts Reclassified from Net Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net loss	1,769	[3]
Prior service cost	93	[3]
Transition obligation	2	[3]
Total, pre-tax	1,864
Tax benefit	(2,503)
Total, net of tax	$ (639)

[1] Included in interest charges.

[2] Included in O&M expenses.

[3] Included in the computation of net periodic pension and post retirement benefit costs. See Note 12 for details regarding these benefit plans.