Commitments and Contingencies - Environmental Matters (Details) $ in Millions	1 Months Ended					6 Months Ended	12 Months Ended
	Jan. 31, 2016	Dec. 31, 2015 USD ($)	Aug. 31, 2014	Apr. 30, 2013	Mar. 31, 2013 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 01, 2015 USD ($)
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 | WE
Air quality
Term of Mercury and Air Toxics Standards compliance extension				1 year
Mercury and Other Hazardous Air Pollutants | Electric | WPS
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						$ 95.0
Steam Electric Effluent Guidelines | Maximum | Electric
Water quality
Expected cost to achieve required emissions reductions						130.0
Manufactured Gas Plant Remediation | Natural gas
Manufactured gas plant remediation
Regulatory assets recorded for remediation of manufactured gas plant sites		$ 697.0				686.6	$ 697.0
Reserves recorded for remediation of manufactured gas plant sites		$ 628.0				$ 617.7	$ 628.0
Weston and Pulliam Consent Decree | 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