Commitments and Contingencies Environmental (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
Sep. 30, 2011
Environmental Remediation Obligation Minimum	$ 29.3
Environmental Remediation Obligation Maximum	38.9
Accrual for Environmental Loss Contingencies	32.2
Manufactured Gas Plants [Member]
Accrual for Environmental Loss Contingencies	27.1
Manufactured Gas Plants [Member] \| Aberdeen South Dakota Site [Member]
Accrual for Environmental Loss Contingencies	13.4
Environmental Remediation Obligation Next 5 years	8.9
Clean Air Act and Emission Control Equipment \| Big Stone Generating Facility [Member] \| Jointly Owned Electricity Generation Plant [Member]
Environmental Obligation Capital Expenditure Estimated	490.0
Jointly Owned Utility Plant Ownership Percentage	23.40%
Clean Air Act and Emission Control Equipment \| Coyote Generating Facility [Member] \| Jointly Owned Electricity Generation Plant [Member]
Environmental Obligation Capital Expenditure Estimated	6.0
Jointly Owned Utility Plant Ownership Percentage	10.00%
Clean Air Act and Emission Control Equipment \| Neal 4 Generating Facility [Member] \| Jointly Owned Electricity Generation Plant [Member]
Environmental Obligation Capital Expenditure Estimated	$ 270.0
Jointly Owned Utility Plant Ownership Percentage	8.70%
Site Contingency, Time Frame of Disbursements	scheduled to commence in 2011 and be spread over the next three years.

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Environmental Remediation Obligation Maximum

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Environmental Remediation Obligation, To be Incurred During Next 5 years

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Total costs accrued as of the balance sheet date for environmental loss contingencies.

+ References

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The proportionate share of interest (percent) in a jointly owned utility plant.

+ References

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-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

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Estimated time frame of disbursements over which the accrued or presently unrecognized amounts may be paid out for an individual site.

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