Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	$ (23,338)
Accumulated other comprehensive income (loss) at end of period	(23,878)		(23,338)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	751,786		762,322		742,975
Total, pre-tax	(698,779)		(704,123)		(690,620)
Income tax expense (benefit)	243,591		250,740		232,544
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(23,338)		(22,871)
Other comprehensive income (loss) before reclassifications	(72)		9
(Gains) losses reclassified from net accumulated other comprehensive loss	(468)		(476)
Net current period other comprehensive income (loss)	(540)		(467)
Accumulated other comprehensive income (loss) at end of period	(23,878)		(23,338)
Gains and Losses on Cash Flow Hedges | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	(755)		(770)
Income tax expense (benefit)	287		294
Total, net of tax	(468)		(476)
Gains and Losses on Cash Flow Hedges | Interest Rate Derivatives | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	(730)	[1]	(730)	[1]
Gains and Losses on Cash Flow Hedges | Vehicle Fuel Derivatives | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	$ (25)	[2]	$ (40)	[2]

[1] Included in interest charges.

[2] Included in O&M expenses