Fixed-asset Impairment (Narrative) (Details) - USD ($) $ in Millions	6 Months Ended		9 Months Ended	12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)	$ 623.5			$ 175.8	$ 859.0	$ 0.0
AES Ohio Generation peakers [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)				109.4	1.3	$ 0.0
Peaking Generating Facilities [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)		$ 4.7
Peaking Generating Facilities [Member] | Fair Value, Inputs, Level 3 [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value		5.2
Killen [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)		230.8		27.3	75.4
Impaired Assets to be Disposed of by Method Other than Sale, Amount of Impairment Loss			$ 6.4
Killen [Member] | Fair Value, Inputs, Level 3 [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value	42.8	$ 84.3		7.9	42.8
Stuart [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)				39.1	228.5
Impaired Assets to be Disposed of by Method Other than Sale, Amount of Impairment Loss			$ 9.8
Stuart [Member] | Fair Value, Inputs, Level 3 [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value	57.4			$ 3.3	57.4
Miami Fort [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)					149.4
Miami Fort [Member] | Fair Value, Inputs, Level 3 [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value	36.5				36.5
Zimmer [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)					144.7
Zimmer [Member] | Fair Value, Inputs, Level 3 [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value	23.7				23.7
Conesville [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)					23.9
Conesville [Member] | Fair Value, Inputs, Level 3 [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value	1.1				1.1
Hutchings Peakers [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)					1.6
Hutchings Peakers [Member] | Fair Value, Inputs, Level 3 [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value	$ 1.6				$ 1.6