Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	3 Months Ended			6 Months Ended
	May 31, 2013	Jun. 30, 2013		Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period		$ (113,620)			$ (112,653)
Other comprehensive loss before reclassifications		(44)			(67)
Losses reclassified from net accumulated other comprehensive loss		1,829			885
Net current period other comprehensive income (loss)		1,785			818
Accumulated other comprehensive loss at end of period		(111,835)			(111,835)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	6,300	146,828		151,921	286,441		303,751
Operating and maintenance expenses		562,557		534,014	1,091,788		1,044,698
Total, pre-tax		(295,775)		(284,876)	(650,797)		(544,160)
Tax benefit		98,893		101,801	217,327		177,316
Total, net of tax		(196,882)		(183,075)	(433,470)		(366,844)
Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, net of tax		1,829			885
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(61,533)			(61,241)
Other comprehensive loss before reclassifications		(44)			(31)
Losses reclassified from net accumulated other comprehensive loss		694			389
Net current period other comprehensive income (loss)		650			358
Accumulated other comprehensive loss at end of period		(60,883)			(60,883)
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		1,145			2,270
Tax benefit		(451)			(1,881)
Total, net of tax		694			389
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		1,162	[1]		2,312	[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		(17)	[2]		(42)	[2]
Unrealized Gains and Losses on Marketable Securities [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(135)			(99)
Other comprehensive loss before reclassifications		0			(36)
Losses reclassified from net accumulated other comprehensive loss		0			0
Net current period other comprehensive income (loss)		0			(36)
Accumulated other comprehensive loss at end of period		(135)			(135)
Defined Benefit Pension and Postretirement Items [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(51,952)			(51,313)
Other comprehensive loss before reclassifications		0			0
Losses reclassified from net accumulated other comprehensive loss		1,135			496
Net current period other comprehensive income (loss)		1,135			496
Accumulated other comprehensive loss at end of period		(50,817)			(50,817)
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		1,769	[3]		3,538	[3]
Prior service cost		93	[3]		186	[3]
Transition obligation		2	[3]		4	[3]
Total, pre-tax		1,864			3,728
Tax benefit		(729)			(3,232)
Total, net of tax		$ 1,135			$ 496

[1] Included in interest charges.

[2] Included in O&M expenses.

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