Derivative Instruments and Hedging Activities (Details) MWh in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021 USD ($) MWh	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2021 USD ($) MWh	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Other Comprehensive Income (Loss), Net of Tax	$ 2,256,000	$ (15,767,000)	$ 29,594,000	$ 5,051,000	$ (2,682,000)	$ (36,313,000)	$ 16,083,000	$ (33,944,000)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	898,000			3,707,000			13,805,000	(36,632,000)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	1,358,000			1,344,000			2,278,000	2,688,000
Accumulated Other Comprehensive Income (Loss), Net of Tax	(27,337,000)						(27,337,000)		$ (43,420,000)	$ (19,750,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	898,000			3,707,000			13,805,000	(36,632,000)
Regulatory assets	$ 84,736,000						$ 84,736,000		45,430,000
AES Indiana energy storage facility MWh planned to acquire	180						180
AES Indiana solar MW planned to acquire	250						250
Regulatory assets	$ 84,736,000						$ 84,736,000		45,430,000
AES Indiana energy storage facility MWh planned to acquire	180						180
FTR [Member] | Not Designated as Hedging Instrument [Member]
Purchase of Units Derivative Instruments Financial Transmission Rights | MWh	9,715						9,715
Sale of Units Derivative Instruments Financial Transmission Rights | MWh	0						0
Derivative, Nonmonetary Notional Amount, Purchase (Sales), Net | MWh	9,715						9,715
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Purchase of Derivative Instruments Interest Rate Swap	$ 400,000,000						$ 400,000,000
Sale of Derivative Instruments Interest Rate Swap	0						0
Derivative, Notional Amount, Purchase (Sales), Net	400,000,000						400,000,000
Interest Rate Contract [Member]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(27,337,000)	$ (29,593,000)		$ (53,694,000)	$ (58,745,000)		(27,337,000)	$ (53,694,000)
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	$ (5,375,000)						$ (5,375,000)
Maximum Length of Time Hedged in Cash Flow Hedge							36 months
Forward power contracts | Not Designated as Hedging Instrument [Member]
Purchase of Units Derivative Instruments Financial Transmission Rights | MWh	163						163
Sale of Units Derivative Instruments Financial Transmission Rights | MWh	0						0
Derivative, Nonmonetary Notional Amount, Purchase (Sales), Net | MWh	163						163
Unrealized Gain (Loss) on Derivatives							$ 300,000
Realized Gain (Loss) on Derivatives	$ 4,900,000						5,600,000
Unrealized Gain (Loss) on Derivatives							300,000
Realized Gain (Loss) on Derivatives	4,900,000						5,600,000
Other Current Assets [Member] | FTR [Member] | Not Designated as Hedging Instrument [Member]
Derivative Asset, Fair Value, Gross Asset									543,000
Other Current Assets [Member] | Forward power contracts | Not Designated as Hedging Instrument [Member]
Derivative Asset, Fair Value, Gross Asset									0
Derivative Liability, Noncurrent | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative Liability, Fair Value, Gross Liability	$ 44,816,000						$ 44,816,000		$ 63,215,000