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 income (loss) at beginning of period	$ (103,366)		$ (111,835)		$ (106,275)		$ (112,653)
Other comprehensive income (loss) before reclassifications	(40)		137		6		70
(Gains) losses reclassified from net accumulated other comprehensive loss	1,405		1,718		4,268		2,603
Net current period other comprehensive income (loss)	1,365		1,855		4,274		2,673
Accumulated other comprehensive income (loss) at end of period	(102,001)		(109,980)		(102,001)		(109,980)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	568,391		575,305		1,714,138		1,667,093
Total, pre-tax	(564,551)		(558,101)		(1,260,965)		(1,208,855)
Tax benefit	195,969		193,349		435,998		410,676
Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, net of tax	1,405		1,718		4,268		2,603
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	(58,610)		(60,883)		(59,753)		(61,241)
Other comprehensive income (loss) before reclassifications	(42)		22		(34)		(9)
(Gains) losses reclassified from net accumulated other comprehensive loss	558		539		1,693		928
Net current period other comprehensive income (loss)	516		561		1,659		919
Accumulated other comprehensive income (loss) at end of period	(58,094)		(60,322)		(58,094)		(60,322)
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	951		805		2,808		3,073
Tax benefit	(393)		(266)		(1,115)		(2,145)
Total, net of tax	558		539		1,693		928
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	967	[1]	829	[1]	2,869	[1]	3,140	[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	(16)	[2]	(24)	[2]	(61)	[2]	(67)	[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	115		(135)		77		(99)
Other comprehensive income (loss) before reclassifications	2		115		40		79
(Gains) losses reclassified from net accumulated other comprehensive loss	0		0		0		0
Net current period other comprehensive income (loss)	2		115		40		79
Accumulated other comprehensive income (loss) at end of period	117		(20)		117		(20)
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	(44,871)		(50,817)		(46,599)		(51,313)
Other comprehensive income (loss) before reclassifications	0		0		0		0
(Gains) losses reclassified from net accumulated other comprehensive loss	847		1,179		2,575		1,675
Net current period other comprehensive income (loss)	847		1,179		2,575		1,675
Accumulated other comprehensive income (loss) at end of period	(44,024)		(49,638)		(44,024)		(49,638)
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	1,500	[3]	1,770	[3]	4,499	[3]	5,308	[3]
Prior service (credit) cost	(86)	[3]	93	[3]	(258)	[3]	279	[3]
Transition obligation	0	[3]	2	[3]	0	[3]	6	[3]
Total, pre-tax	1,414		1,865		4,241		5,593
Tax benefit	(567)		(686)		(1,666)		(3,918)
Total, net of tax	$ 847		$ 1,179		$ 2,575		$ 1,675

[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.