Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period					$ (23,338)
Accumulated other comprehensive loss at end of period	(23,709)				(23,709)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	185,510		192,073		549,935		553,266
Total, pre-tax	(237,132)		(259,502)		(557,716)		(591,721)
Tax expense	82,973		93,236		195,362		211,551
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(23,573)		(23,120)		(23,338)		(22,871)
Other comprehensive income (loss) before reclassifications	(17)		11		(15)		(2)
Gains reclassified from net accumulated other comprehensive loss	(119)		(119)		(356)		(355)
Net current period other comprehensive loss	(136)		(108)		(371)		(357)
Accumulated other comprehensive loss at end of period	(23,709)		(23,228)		(23,709)		(23,228)
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	(191)		(195)		(573)		(576)
Tax expense	72		76		217		221
Total, net of tax	(119)		(119)		(356)		(355)
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	(184)	[1]	(184)	[1]	(546)	[1]	(546)	[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	$ (7)	[2]	$ (11)	[2]	$ (27)	[2]	$ (30)	[2]

[1] Included in interest charges.

[2] Included in O&M expenses