Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	$ (108,139)		$ (106,275)
Other comprehensive income (loss) before reclassifications	(10)		31
(Gains) losses reclassified from net accumulated other comprehensive loss	1,461		1,424
Net current period other comprehensive income (loss)	1,451		1,455
Accumulated other comprehensive income (loss) at end of period	(106,688)		(104,820)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	585,830		560,143
Total, pre-tax	(235,646)		(396,992)
Tax benefit	83,580		135,771
Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, net of tax	1,461		1,424
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	(57,628)		(59,753)
Other comprehensive income (loss) before reclassifications	(11)		(7)
(Gains) losses reclassified from net accumulated other comprehensive loss	585		560
Net current period other comprehensive income (loss)	574		553
Accumulated other comprehensive income (loss) at end of period	(57,054)		(59,200)
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	967		918
Tax benefit	(382)		(358)
Total, net of tax	585		560
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	941	[1]	946	[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	26	[2]	(28)	[2]
Unrealized Gains and Losses on Marketable Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	110		77
Other comprehensive income (loss) before reclassifications	1		38
(Gains) losses reclassified from net accumulated other comprehensive loss	0		0
Net current period other comprehensive income (loss)	1		38
Accumulated other comprehensive income (loss) at end of period	111		115
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(50,621)		(46,599)
Other comprehensive income (loss) before reclassifications	0		0
(Gains) losses reclassified from net accumulated other comprehensive loss	876		864
Net current period other comprehensive income (loss)	876		864
Accumulated other comprehensive income (loss) at end of period	(49,745)		(45,735)
Defined Benefit Pension and Postretirement Items | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net loss	1,535	[3]	1,500	[3]
Prior service (credit) cost	(90)	[3]	(86)	[3]
Transition obligation	0	[3]	0	[3]
Total, pre-tax	1,445		1,414
Tax benefit	(569)		(550)
Total, net of tax	$ 876		$ 864

[1] Included in interest charges.

[2] Included in O&M expenses.

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