Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive gain (loss) at beginning of period	$ (106,275)		$ (112,653)
Other comprehensive gain (loss) before reclassifications	31		(23)
(Gains) losses reclassified from net accumulated other comprehensive loss	1,424		(944)
Net current period other comprehensive income (loss)	1,455		(967)
Accumulated other comprehensive gain (loss) at end of period	(104,820)		(113,620)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	560,143		529,231
Total, pre-tax	(396,992)		(355,004)
Tax benefit	135,771		118,434
Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, net of tax	1,424		(944)
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive gain (loss) at beginning of period	(59,753)		(61,241)
Other comprehensive gain (loss) before reclassifications	(7)		13
(Gains) losses reclassified from net accumulated other comprehensive loss	560		(305)
Net current period other comprehensive income (loss)	553		(292)
Accumulated other comprehensive gain (loss) at end of period	(59,200)		(61,533)
Gains and Losses on Cash Flow Hedges [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	918		1,124
Tax benefit	(358)		(1,429)
Total, net of tax	560		(305)
Gains and Losses on Cash Flow Hedges [Member] | Interest Rate Derivatives [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	946	[1]	1,150	[1]
Gains and Losses on Cash Flow Hedges [Member] | Vehicle Fuel And Other Commodity Derivatives [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	(28)	[2]	(26)	[2]
Unrealized Gains and Losses on Marketable Securities [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive gain (loss) at beginning of period	77		(99)
Other comprehensive gain (loss) before reclassifications	38		(36)
(Gains) losses reclassified from net accumulated other comprehensive loss	0		0
Net current period other comprehensive income (loss)	38		(36)
Accumulated other comprehensive gain (loss) at end of period	115		(135)
Defined Benefit Pension and Postretirement Items [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive gain (loss) at beginning of period	(46,599)		(51,313)
Other comprehensive gain (loss) before reclassifications	0		0
(Gains) losses reclassified from net accumulated other comprehensive loss	864		(639)
Net current period other comprehensive income (loss)	864		(639)
Accumulated other comprehensive gain (loss) at end of period	(45,735)		(51,952)
Defined Benefit Pension and Postretirement Items [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net loss	1,500	[3]	1,769	[3]
Prior service (credit) cost	(86)	[3]	93	[3]
Transition obligation	0	[3]	2	[3]
Total, pre-tax	1,414		1,864
Tax benefit	(550)		(2,503)
Total, net of tax	$ 864		$ (639)

[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.