Commitments and Contingencies - Environmental Matters (Details) $ in Millions	1 Months Ended						12 Months Ended
	Sep. 30, 2016 USD ($)	Jan. 31, 2016	Dec. 31, 2015 USD ($)	Aug. 31, 2014	Apr. 30, 2013	Mar. 31, 2013 USD ($)	Dec. 31, 2015 USD ($)	Jun. 01, 2015 USD ($)
Cross-State Air Pollution Rule | Electric
Air quality
Number of states impacted by Cross-State Air Pollution Rule	23
Mercury and Other Hazardous Air Pollutants | Electric
Air quality
Percentage of mercury emissions reduction required by Wisconsin and Michigan	90.00%
Mercury and Other Hazardous Air Pollutants | Electric | Michigan | WE
Air quality
Term of Mercury and Air Toxics Standards compliance extension					1 year
Climate Change | Electric
Air quality
Percentage of nationwide greenhouse gas emissions reduction							32.00%
Interim goal for greenhouse gas emissions reduction (fraction)							0.667
Climate Change | Electric | Wisconsin
Air quality
Percentage of greenhouse gas emissions reduction by state							41.00%
Percentage of greenhouse gas emissions reduction for retirement of a nuclear plant			10.00%
Climate Change | Electric | Michigan
Air quality
Percentage of greenhouse gas emissions reduction by state							39.00%
Clean Water Act Cooling Water Intake Structure Rule | Electric
Water quality
Number of compliance options available to meet standard				7
Steam Electric Effluent Guidelines | Electric
Water quality
Renewal period for facility permits		5 years
Steam Electric Effluent Guidelines | Minimum | Electric
Water quality
Expected cost to achieve required emissions reductions	$ 80.0
Steam Electric Effluent Guidelines | Maximum | Electric
Water quality
Expected cost to achieve required emissions reductions	110.0
Manufactured Gas Plant Remediation | Natural gas
Manufactured gas plant remediation
Regulatory assets recorded for remediation of manufactured gas plant sites	681.8		$ 697.0				$ 697.0
Reserves recorded for remediation of manufactured gas plant sites	$ 608.1		$ 628.0				$ 628.0
Wisconsin Public Service Corporation Consent Decree - Weston and Pulliam | Electric | WPS
WPS consent decree - Weston and Pulliam
Beneficial environmental projects						$ 6.0
Regulatory asset for undepreciated book value of retired plants								$ 11.5
Civil penalty						$ 1.2