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	$ (22,871)
Accumulated other comprehensive loss at end of period	(22,982)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	41,388		48,291
Operating and maintenance expenses	173,041		168,382
Total, pre-tax	(180,562)		(145,534)
Tax benefit	63,957		52,249
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss at beginning of period	(22,871)
Other comprehensive income (loss) before reclassifications	7
(Gains) losses reclassified from net accumulated other comprehensive loss	(118)
Net current period other comprehensive loss	(111)
Accumulated other comprehensive loss at end of period	(22,982)
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	(192)
Tax benefit	74
Total, net of tax	(118)
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	(180)	[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	$ (12)	[2]

[1] Included in interest charges.

[2] Included in O&M expenses.