Adopted Reduction Related to the Ozone Season Emission Budget for NOx

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Former Level of Air Quality Concentrations (in parts per billion)

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Length of Time States Are Required to Revise Plans (in Years)

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Estimated undiscounted amount of the accrual required to comply with required regulation.

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Estimated undiscounted amount of the accrual required to comply with required entrainment regulation under the federal Clean Water Act Section 316(b) rule.

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Minimum number of plants which could be required by state regulators to make improvements to reduce entrainment

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Number of hours measured for standard

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Number of phases under a consent decree which the EPA is requiring states to evaluate areas for attainment

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Number of years unclassifiable areas will be monitored

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Revised Level of Air Quality Concentrations (in parts per billion)

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Commitments and Contingencies, Environmental Contingencies - Unrecorded Unconditional Purchase Obligation (Details) $ in Millions	1 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2016 USD ($) Plant	Dec. 31, 2015 Period
Regional Haze Rules
Environmental Requirements [Abstract]
Length of Time States Are Required to Revise Plans (in Years)		10 years
Implementation of the National Ambient Air Quality Standard for Sulfur Dioxide
Environmental Requirements [Abstract]
Number of Phases Under a Consent Decree Which the EPA is Requiring States to Evaluate Areas for Attainment		3
National Ambient Air Quality Standards for Ozone
Environmental Requirements [Abstract]
Number of Hours Measured for Standard \| Period			8
Former Level of Air Quality Concentrations (in parts per billion)			75
Revised Level of Air Quality Concentrations (in parts per billion)			70
NSP-Minnesota \| Federal Clean Water Act Section 316(b)
Environmental Requirements [Abstract]
Minimum Number of Plants Which Could Be Required to Make Improvements to Reduce Entrainment \| Plant		6
NSP-Wisconsin \| Federal Clean Water Act Section 316(b)
Environmental Requirements [Abstract]
Minimum Number of Plants Which Could Be Required to Make Improvements to Reduce Entrainment \| Plant		2
SPS \| Cross-State Air Pollution Rule (CSAPR)
Environmental Requirements [Abstract]
Adopted Reduction Related to the Ozone Season Emission Budget for NOx	22.00%
SPS \| National Ambient Air Quality Standards for Ozone
Environmental Requirements [Abstract]
Revised Level of Air Quality Concentrations (in parts per billion)		70
Capital Commitments \| Federal Clean Water Act Section 316(b)
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply with Regulation		$ 53
Minimum \| Capital Commitments \| Federal Clean Water Act Effluent Limitations Guidelines
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply with Regulation		9
Maximum \| Capital Commitments \| Federal Clean Water Act Effluent Limitations Guidelines
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply with Regulation		21
Maximum \| Capital Commitments \| Federal Clean Water Act Section 316(b)
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply With Entrainment Regulation		$ 192
Harrington Units 1 and 2 \| Implementation of the National Ambient Air Quality Standard for Sulfur Dioxide
Environmental Requirements [Abstract]
Number of Years Unclassifiable Areas Will Be Monitored		3 years
Harrington Units 1 and 2 \| Capital Commitments \| SPS \| Regional Haze Rules
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply with Regulation		$ 400
Tolk Units 1 and 2 \| Capital Commitments \| SPS \| Regional Haze Rules
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply with Regulation		$ 600