Regulatory Matters - Ohio (Details) MWh in Thousands, $ in Millions			3 Months Ended
	Aug. 07, 2013 USD ($) auctions	Mar. 20, 2013	Mar. 31, 2016 USD ($) MWh MW	Mar. 31, 2015 USD ($)	Apr. 15, 2016 USD ($)
Regulatory Matters [Line Items]
Proposed goal to reduce CO2 pollution (percent)			90.00%
Amortization (deferral) of regulatory assets, net			$ 61.0	$ 32.0
Ohio
Regulatory Matters [Line Items]
ESP extension term			2 years
Generation discount for low income customers (percent)			6.00%
Recovery period			5 years
Costs avoided by customers			$ 360.0
Proposed goal to reduce CO2 pollution (percent)			90.00%
Generation supply auction period, after approval			3 years
Generation supply auction period, before approval			1 year
Term of proposed purchase power agreement			8 years
Term of economic development and assistance plan			3 years
Annual revenue cap for rider			$ 30.0
Guaranteed credits, year seven			$ 30.0
Energy efficient portfolio plan term		3 years
Credit to non-shopping customers	$ 43.4
Ohio | Year 2015
Regulatory Matters [Line Items]
Annual energy savings (in GWH) | MWh			2,266
Ohio | Year 2016
Regulatory Matters [Line Items]
Annual energy savings (in GWH) | MWh			2,288
Ohio | Year 2017
Regulatory Matters [Line Items]
Annual increase in energy savings (percent)			1.00%
Ohio | Annually Through 2020
Regulatory Matters [Line Items]
Utilities required to additionally reduce peak demand			0.75%
Ohio | Wind or Solar Power
Regulatory Matters [Line Items]
Proposed potential plant acquisition (MW) | MW			100
Ohio | PUCO
Regulatory Matters [Line Items]
Number of renewable energy auctions | auctions	1
Approved period to limit average customer bill			2 years
Ohio | Ohio Companies | PUCO | Subsequent Event
Regulatory Matters [Line Items]
Expenditures for cost recovery program					$ 323.0
Ohio | Retail Rate Stability Rider
Regulatory Matters [Line Items]
Guaranteed credits, year five			$ 10.0
Guaranteed credits, year six			20.0
Guaranteed credits, year eight			$ 40.0
Ohio | Retail Rate Stability Rider | FES
Regulatory Matters [Line Items]
Term of proposed purchase power agreement			8 years
Ohio | Delivery Capital Recovery Rider
Regulatory Matters [Line Items]
Annual revenue cap for rider			$ 30.0
Annual revenue cap for rider for years three through six			20.0
Annual revenue cap for rider for years six through eight			$ 15.0
Ohio | Customary Advisory Council
Regulatory Matters [Line Items]
Term of proposed purchase power agreement			8 years
Annual contribution amount			$ 1.0
Contribution amount			$ 8.0
Ohio | Energy Conservation, Economic Development and Job Retention | Ohio Companies
Regulatory Matters [Line Items]
Term of proposed purchase power agreement			8 years
Annual contribution amount			$ 3.0
Contribution amount			$ 24.0
Ohio | Fuel-Fund | Ohio Companies
Regulatory Matters [Line Items]
Term of proposed purchase power agreement			8 years
Annual contribution amount			$ 2.4
Contribution amount			$ 19.0