Overview and Summary of Significant Accounting Policies (Details) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021 USD ($) generating_station MW	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2021 USD ($) generating_station MW	Sep. 30, 2020 USD ($)	May 31, 2021	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 536,681	$ 526,680	$ 558,531	$ 511,659	$ 507,839	$ 518,761	$ 536,681	$ 511,659		$ 520,988	$ 546,476
Accumulated Other Comprehensive Income (Loss), Net of Tax	(27,337)						(27,337)			(43,420)	(19,750)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	898			3,707			13,805	(36,632)
Inventories	17,230						17,230			36,953
Cash and cash equivalents	$ 13,126						$ 13,126			20,502
Number of customers	515,000						515,000
Number of generating stations | generating_station	4						4
Restricted Cash and Cash Equivalents	$ 5						$ 5			6,120
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	13,131			59,328			13,131	59,328		26,622	48,552
Regulatory assets, current	59,132						59,132			58,553
Inventory, Net	76,362						76,362			95,506
Other Comprehensive Income (Loss), Net of Tax	2,256	(15,767)	$ 29,594	5,051	(2,682)	$ (36,313)	16,083	(33,944)
Accounts Receivable, Allowance for Credit Loss, Current	1,100			3,520			1,100	3,520		3,155	921
Accounts Receivable, Credit Loss Expense (Reversal)							1,333	4,366
Accounts Receivable, Allowance for Credit Loss, Writeoff							(4,903)	(3,127)
Accounts Receivable, Allowance for Credit Loss, Recovery							1,515	1,360
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(27,337)						(27,337)			$ (43,420)	$ (19,750)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	898			3,707			13,805	(36,632)
Income Tax Expense (Benefit)	10,966			10,981			25,506	25,918
Income Tax Expense (Benefit)	10,966			10,981			25,506	25,918
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	$ 1,358			1,344			$ 2,278	2,688
Petersburg Unit 1 retired MW									230
Petersburg Unit 2 Planned Retirement MW in 2023							415
Preferred Resource Portfolio Coal-Fired Generation Retirement MW by 2023	630						630
Indianapolis Power And Light Company [Member]
Debt Instrument [Line Items]
Electric generation capability for winter, megawatts | MW	3,475						3,475
Electric generation capability for summer, megawatts | MW	3,330						3,330
Aes U.S. Investments [Member]
Debt Instrument [Line Items]
Ownership percentage by parent (percent)	82.35%						82.35%
CDPQ [Member]
Debt Instrument [Line Items]
Ownership percentage by parent (percent)	17.65%						17.65%
Ownership interest in parent company (percent)	15.00%						15.00%
AES U.S. Holdings, LLC [Member]
Debt Instrument [Line Items]
Ownership interest in parent company (percent)	85.00%						85.00%
Harding Street [Member]
Debt Instrument [Line Items]
Amount of New Operation for Battery Storage Unit, megawatts | MW	20						20
Interest Rate Contract [Member]
Debt Instrument [Line Items]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	$ (27,337)	$ (29,593)		(53,694)	$ (58,745)		$ (27,337)	(53,694)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Debt Instrument [Line Items]
Interest and Debt Expense	(1,807)			(1,807)			(3,011)	(3,614)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest and Debt Expense	(1,807)			(1,807)			(3,011)	(3,614)
Income Tax Expense (Benefit)	449			463			733	926
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	1,358			1,344			2,278	2,688
Income Tax Expense (Benefit)	449			463			733	926
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 1,358			$ 1,344			$ 2,278	$ 2,688