Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 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	(23,461)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	175,524		173,041
Total, pre-tax	(180,900)		(180,562)
Tax expense	62,497		63,957
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(23,338)		(22,871)
Other comprehensive gain (loss) before reclassifications	(3)		7
Gains reclassified from net accumulated other comprehensive loss	(120)		(118)
Net current period other comprehensive loss	(123)		(111)
Accumulated other comprehensive loss at end of period	(23,461)		(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	(193)		(192)
Tax expense	73		74
Total, net of tax	(120)		(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]	(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	$ (13)	[2]	$ (12)	[2]

[1] Included in interest charges.

[2] Included in O&M expenses