Regulatory Matters (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	May 31, 2021
Regulatory Matters [Line Items]
Total current regulatory assets	$ 84,736	$ 45,430
Total long-term regulatory assets	364,975	392,801
Total current regulatory liabilities	7,427	30,036
Total long-term regulatory liabilities	828,803	839,360
COVID-19 related uncollectible and incremental bad debt expense	$ 5,400	$ 6,400
Preferred Resource Portfolio Coal-Fired Generation Retirement MW by 2023	630
Solar Project Acquisition MW Expected Completion in 2023	195
Estimated Projected Costs for Successful Proposed EV Portfolio Implementation	$ 5,100
Period of Time for Implementation of Services Propsed in the Electric Vehicle (EV) Portfolio	3
AES Indiana solar MW planned to acquire	250
AES Indiana energy storage facility MWh planned to acquire	180
Petersburg Unit 1 retired MW			230
AES Indiana solar MW planned to acquire	250
AES Indiana energy storage facility MWh planned to acquire	180
Petersburg Unit 1 retirement
Regulatory Matters [Line Items]
Total long-term regulatory assets	$ 63,100