Income Taxes (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009	Dec. 31, 2007
Corporate Owned Life Insurance [Abstract]
Payment in settlement of U.S. government's claims for tax, penalty, and interest related to COLI program							$ 64,400,000
Net non-recurring tax and interest charge		(300,000)		9,400,000
Final cash settlement received		700,000
Amount received as full and final settlement				25,000,000
Medicare Part D Subsidy Reimbursements [Abstract]
Previously recognized tax benefits reversed	17,000,000
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit - Permanent tax positions		4,300,000		5,900,000
Unrecognized tax benefit - Temporary tax positions		30,400,000		34,600,000
Unrecognized tax benefit balance		34,700,000		40,500,000		30,300,000
Reconciliation of Unrecognized Tax Benefit [Roll Forward]
Balance at Jan. 1	30,300,000	40,500,000		30,300,000		42,100,000
Additions based on tax positions related to the current year - continuing operations		11,900,000		13,400,000		12,600,000
Reductions based on tax positions related to the current year - continuing operations		(1,900,000)		(600,000)		(1,800,000)
Additions for tax positions of prior years - continuing operations		14,000,000		5,500,000		6,800,000
Reductions for tax positions of prior years - continuing operations		(2,400,000)		(1,800,000)		(2,300,000)
Reductions for tax positions of prior years - discontinued operations		0		(6,300,000)		0
Settlements with taxing authorities - continuing operations		(27,300,000)		0		(27,100,000)
Lapse of applicable statutes of limitations - continuing operations		(100,000)		0		0
Balance at Dec. 31		34,700,000		40,500,000		30,300,000
Tax Benefits Associated with NOL and Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards		(33,600,000)		(38,000,000)
Decrease in unrecognized tax benefit		(5,800,000)
Reconciliation of Interest Payable Related to Unrecognized Tax Benefit [Roll Forward]
Payable for interest related to unrecognized tax benefits at Jan. 1	(200,000)	(300,000)		(200,000)		(400,000)
Interest income (expense) related to unrecognized tax benefits - continuing operations		900,000		(600,000)		1,500,000
Interest (expense) income related to unrecognized tax benefits - discontinued operations		(800,000)		500,000		(1,300,000)
Payable for interest related to unrecognized tax benefits at Dec. 31		(200,000)		(300,000)		(200,000)
Amounts accrued for penalties related to unrecognized tax benefits		0		0		0
Effective Tax Rate Reconciliation [Abstract]
Federal statutory rate (in hundredths)		35.00%		35.00%		35.00%
Increases (Decreases) in Tax From [Abstract]
State income taxes, net of federal income tax benefit (in hundredths)		4.20%		3.90%		4.00%
Resolution of income tax audits and other (in hundredths)		0.30%		0.60%		0.80%
Tax credits recognized, net of federal income tax expense (in hundredths)		(2.60%)		(1.80%)		(2.00%)
Regulatory differences - utility plant items (in hundredths)		(0.80%)		(1.10%)		(2.00%)
Change in unrecognized tax benefits (in hundredths)		(0.10%)		0.10%		(0.50%)
Life insurance policies (in hundredths)		(0.10%)		(0.80%)		(0.20%)
Previously recognized Medicare Part D subsidies (in hundredths)		0.00%		1.40%		0.00%
Other, net (in hundredths)		(0.10%)		(0.60%)		0.00%
Effective income tax rate from continuing operations (in hundredths)		35.80%		36.70%		35.10%
Components of Income Tax Expense from Continuing Operations [Abstract]
Current federal tax expense (benefit)		3,399,000		16,657,000		(39,886,000)
Current state tax expense		9,971,000		12,580,000		8,672,000
Current change in unrecognized tax benefit		(8,266,000)		(2,982,000)		(7,627,000)
Current tax credits		0		(944,000)		0
Deferred federal tax expense		410,794,000		376,073,000		360,252,000
Deferred state tax expense		80,670,000		52,543,000		69,947,000
Deferred change in unrecognized tax expense		6,705,000		4,641,000		2,387,000
Deferred tax credits		(28,763,000)		(15,580,000)		(16,005,000)
Deferred investment tax credits		(6,194,000)		(6,353,000)		(6,426,000)
Total income tax expense from continuing operation		468,316,000		436,635,000		371,314,000
Deferred Tax Liabilities [Abstract]
Differences between book and tax bases of property		4,558,951,000		3,853,425,000
Regulatory assets		253,162,000		242,760,000
Other		279,162,000		219,035,000
Total deferred tax liabilities		5,091,275,000		4,315,220,000
Deferred Tax Assets [Abstract]
NOL carryforward		696,435,000		425,620,000
Tax credit carryforward		254,157,000		226,057,000
Unbilled revenue - fuel costs		73,912,000		69,358,000
Environmental remediation		45,551,000		41,696,000
Rate refund		37,443,000		8,971,000
Deferred investment tax credits		37,425,000		39,916,000
Regulatory liabilities		37,012,000		51,600,000
Accrued liabilities and other		73,092,000		58,891,000
NOL and tax credit valuation allowances		(5,683,000)		(1,927,000)
Total deferred tax assets		1,249,344,000		920,182,000
Net deferred tax liability		3,841,931,000		3,395,038,000
Internal Revenue Service (IRS) [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
NOL carryforward		1,710,000,000		989,000,000
Federal tax credit carryforwards		232,000,000		205,000,000
Carryforward expiration date range, low		2021
Carryforward expiration date range, high		2031
State Jurisdiction [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
NOL carryforward		1,707,000,000		1,363,000,000
Valuation allowances for state NOL carryforwards		(51,000,000)		(32,000,000)
State tax credit carryforwards, net of federal detriment		22,000,000	[1]	21,000,000	[1]
Valuation allowances for state tax credit carryforwards, net of federal benefit		(2,000,000)		0
Carryforward expiration date range, low		2012
Carryforward expiration date range, high		2031
Federal detriment		12,000,000		11,000,000
Internal Revenue Service (IRS) [Member]
Income Tax Examination [Line Items]
Total assessment for tax years 2008 and 2009, including tax and interest		$ 1,400,000
Year(s) examination completed		2008 and 2009
Year(s) no longer subject to audit as statute of limitations has expired		2007
Earliest year subject to examination		2008
Year(s) under examination		None
Colorado [Member]
Income Tax Examination [Line Items]
Earliest year subject to examination		2006
Year(s) under examination		None
Minnesota [Member]
Income Tax Examination [Line Items]
Earliest year subject to examination		2007
Year(s) under examination		None
Texas [Member]
Income Tax Examination [Line Items]
Earliest year subject to examination		2007
Year(s) under examination		None
Wisconsin [Member]
Income Tax Examination [Line Items]
Earliest year subject to examination		2007
Year(s) under examination		None

[1] State tax credit carryforwards are net of federal detriment of $12 million and $11 million as of Dec. 31, 2011 and 2010, respectively.