Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive gain (loss) at beginning of period	$ (104,820)		$ (113,620)		$ (106,275)		$ (112,653)
Other comprehensive gain (loss) before reclassifications	16		(44)		46		(67)
(Gains) losses reclassified from net accumulated other comprehensive loss	1,438		1,829		2,863		885
Net current period other comprehensive income (loss)	1,454		1,785		2,909		818
Accumulated other comprehensive gain (loss) at end of period	(103,366)		(111,835)		(103,366)		(111,835)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	585,604		562,557		1,145,747		1,091,788
Total, pre-tax	(299,422)		(295,750)		(696,414)		(650,754)
Tax benefit	104,258		98,893		240,029		217,327
Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, net of tax	1,438		1,829		2,863		885
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive gain (loss) at beginning of period	(59,200)		(61,533)		(59,753)		(61,241)
Other comprehensive gain (loss) before reclassifications	16		(44)		8		(31)
(Gains) losses reclassified from net accumulated other comprehensive loss	574		694		1,135		389
Net current period other comprehensive income (loss)	590		650		1,143		358
Accumulated other comprehensive gain (loss) at end of period	(58,610)		(60,883)		(58,610)		(60,883)
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	939		1,145		1,857		2,270
Tax benefit	(365)		(451)		(722)		(1,881)
Total, net of tax	574		694		1,135		389
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	956	[1]	1,162	[1]	1,902	[1]	2,312	[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	(17)	[2]	(17)	[2]	(45)	[2]	(42)	[2]
Unrealized Gains and Losses on Marketable Securities
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive gain (loss) at beginning of period	115		(135)		77		(99)
Other comprehensive gain (loss) before reclassifications	0		0		38		(36)
(Gains) losses reclassified from net accumulated other comprehensive loss	0		0		0		0
Net current period other comprehensive income (loss)	0		0		38		(36)
Accumulated other comprehensive gain (loss) at end of period	115		(135)		115		(135)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive gain (loss) at beginning of period	(45,735)		(51,952)		(46,599)		(51,313)
Other comprehensive gain (loss) before reclassifications	0		0		0		0
(Gains) losses reclassified from net accumulated other comprehensive loss	864		1,135		1,728		496
Net current period other comprehensive income (loss)	864		1,135		1,728		496
Accumulated other comprehensive gain (loss) at end of period	(44,871)		(50,817)		(44,871)		(50,817)
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]	1,769	[3]	2,999	[3]	3,538	[3]
Prior service (credit) cost	(86)	[3]	93	[3]	(172)	[3]	186	[3]
Transition obligation	0	[3]	2	[3]	0	[3]	4	[3]
Total, pre-tax	1,414		1,864		2,827		3,728
Tax benefit	(550)		(729)		(1,099)		(3,232)
Total, net of tax	$ 864		$ 1,135		$ 1,728		$ 496

[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.