Commitments, Guarantees and Contingencies - Clean Air Act and Climate Change (Details) $ in Millions					3 Months Ended	12 Months Ended
	Feb. 18, 2018 USD ($)	May 01, 2017 USD ($)	Oct. 01, 2015	Aug. 03, 2015 T	Mar. 31, 2017 USD ($)	Dec. 31, 2017 phase T	Nov. 12, 2014
Loss Contingencies [Line Items]
Proposed reduction in power plants carbon pollution (percent)						90.00%
National Ambient Air Quality Standards
Loss Contingencies [Line Items]
Capping of SO2 Emissions Under CSAPR | T						2,400,000
Capping of NOx emissions under CSAPR | T						1,200,000
National Ambient Air Quality Standards | CSAPR
Loss Contingencies [Line Items]
Number of phases under the EPA’s CAIR for reductions of Sulfur Dioxide and Mono-Nitrogen Oxides | phase						2
Mercury and Air Toxic Standards | FG | Certain Coal-Fired Power Plant
Loss Contingencies [Line Items]
Contractual amount in dispute (in T) | T				3,500,000.0
Mercury and Air Toxic Standards | FG | Another Coal-Fired Power Plant
Loss Contingencies [Line Items]
Contractual amount in dispute (in T) | T				2,500,000.0
Minimum | Climate Change
Loss Contingencies [Line Items]
Reduction in emissions (percent)							26.00%
Maximum | Climate Change
Loss Contingencies [Line Items]
Reduction in emissions (percent)							28.00%
EPA | Caa Compliance
Loss Contingencies [Line Items]
Period of time to implement plan			3 years
Signal Peak, Global Rail and Affiliates | Senior Secured Term Loan | Senior Loans | Global Holding
Loss Contingencies [Line Items]
Investment ownership percentage						69.99%
Settled Litigation | Caa Compliance
Loss Contingencies [Line Items]
Loss in period					$ 116
Settled Litigation | Caa Compliance | FG
Loss Contingencies [Line Items]
Amount of damages awarded		$ 109
Settled Litigation | Caa Compliance | FES
Loss Contingencies [Line Items]
Loss in period					$ 116
Subsequent Event
Loss Contingencies [Line Items]
Amount of damages awarded to other party	$ 93
Subsequent Event | AE Supply
Loss Contingencies [Line Items]
Amount of damages awarded to other party	$ 93