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Variable Interest Entities (Schedule of Cash Contributions) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Cash Contributions Noncontrolling Interest [Line Items]      
Obligation to the Developer $ 0.0 $ 0.0 $ 277.5
Consolidated Variable Interest Entities      
Cash Contributions Noncontrolling Interest [Line Items]      
NIPSCO's Cash Contribution 401.5    
Tax Equity Partners Cash Contribution $ 507.2    
Obligation to the Developer   $ 347.2