Income Taxes (Details) (USD $)	12 Months Ended				12 Months Ended		12 Months Ended		3 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2010 Provision	Dec. 31, 2012 Internal Revenue Service (IRS) [Member]	Dec. 31, 2011 Internal Revenue Service (IRS) [Member]	Dec. 31, 2012 State Jurisdiction [Member]	Dec. 31, 2011 State Jurisdiction [Member]	Sep. 30, 2012 Internal Revenue Service (IRS) [Member]	Dec. 31, 2012 Internal Revenue Service (IRS) [Member]	Dec. 31, 2012 State Jurisdiction [Member]	Dec. 31, 2012 State Jurisdiction [Member]
Prescription Drug Tax Benefit and Medicare Part D [Abstract]
Income tax expense (benefit) related to retiree prescription drug benefits	$ (17,000,000)
Number of applicable provisions addressing deductibility of retiree health care costs				1
Tax Audits [Abstract]
Year(s) no longer subject to audit as statute of limitations has expired										2008
Earliest year subject to examination										2009	2006	2006
Year(s) under examination									2010 and 2011		2006 through 2009	None
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit - Permanent tax positions	1,300,000	500,000
Unrecognized tax benefit - Temporary tax positions	8,300,000	10,900,000
Total unrecognized tax benefit	9,600,000	11,400,000	11,600,000
Reconciliation of Unrecognized Tax Benefits [Roll Forward]
Balance at Jan. 1	11,400,000	11,600,000	7,200,000
Additions based on tax positions related to the current year	1,900,000	3,400,000	4,100,000
Reductions based on tax positions related to the current year	(1,500,000)	(800,000)	(200,000)
Additions for tax positions of prior years	2,000,000	5,800,000	1,600,000
Reductions for tax positions of prior years	(4,200,000)	(900,000)	(1,100,000)
Settlements with taxing authorities	0	(7,700,000)	0
Balance at Dec. 31	9,600,000	11,400,000	11,600,000
Tax Benefits Associated With Nol And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards	(5,300,000)	(3,700,000)
Amounts accrued for penalties related to unrecognized tax benefits	0	0	0
Operating Loss and Tax Credit Carryforwards [Line Items]
NOL carryforward					264,700,000	391,600,000	595,100,000	566,800,000
Federal tax credit carryforwards					16,200,000	15,500,000
State tax credit carryforwards, net of federal detriment							11,600,000	9,400,000
Carryforward expiration date range, low					2021		2015
Carryforward expiration date range, high					2031		2031
Effective Tax Rate Reconciliation [Abstract]
Federal statutory rate (in hundredths)	35.00%	35.00%	35.00%
Increases (Decreases) in Tax From [Abstract]
Prescription drug tax benefit and Medicare Part D (in hundredths)	(2.50%)	(0.10%)	1.60%
Tax credits recognized, net of federal income tax expense (in hundredths)	(0.60%)	(0.80%)	(0.80%)
Regulatory differences - utility plant items (in hundredths)	(0.60%)	(0.20%)	(0.50%)
State income taxes, net of federal income tax benefit (in hundredths)	2.30%	2.50%	1.00%
Change in unrecognized tax benefits (in hundredths)	0.10%	(0.10%)	0.00%
Life insurance policies (in hundredths)	0.00%	0.00%	(1.40%)
Other, net (in hundredths)	0.00%	0.20%	1.50%
Effective income tax rate from continuing operations (in hundredths)	33.70%	36.50%	36.40%
Components of Income Tax Expense [Abstract]
Current federal tax expense	176,354,000	1,889,000	76,228,000
Current state tax expense (benefit)	24,502,000	(796,000)	(461,000)
Current change in unrecognized tax (benefit) expense	(3,447,000)	3,326,000	1,246,000
Deferred federal tax expense	38,309,000	207,620,000	146,501,000
Deferred state tax (benefit) expense	(4,424,000)	22,994,000	9,280,000
Deferred change in unrecognized tax expense (benefit)	4,207,000	(4,059,000)	(920,000)
Deferred investment tax credits	(2,957,000)	(2,613,000)	(2,693,000)
Total income tax expense	232,544,000	228,361,000	229,181,000
Components of Deferred Income Tax Expense [Abstract]
Deferred tax expense excluding items below	41,233,000	216,393,000	160,543,000
Tax benefit allocated to other comprehensive income	6,433,000	12,149,000	393,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities	(9,574,000)	(1,987,000)	(6,075,000)
Deferred tax expense	38,092,000	226,555,000	154,861,000
Deferred Tax Liabilities [Abstract]
Differences between book and tax bases of property	1,838,065,000	1,823,058,000
Employee benefits	99,286,000	130,544,000
Other	128,453,000	113,489,000
Total deferred tax liabilities	2,065,804,000	2,067,091,000
Deferred Tax Assets [Abstract]
NOL carryforward	129,829,000	169,688,000
Unbilled revenue - fuel costs	54,353,000	61,118,000
Tax credit carryforward	27,752,000	24,972,000
Deferred investment tax credits	15,992,000	16,985,000
Regulatory liabilities	14,164,000	14,693,000
Other	40,382,000	37,536,000
Total deferred tax assets	282,472,000	324,992,000
Net deferred tax liability	$ 1,783,332,000	$ 1,742,099,000