Revenue Recognition Revenue Recognition (Allowance for Credit Losses) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2022
Accounts Receivable Allowance For Credit Loss [Line Items]
Accounts Receivable, Allowance for Credit Loss	$ 26.2	$ 22.9	$ 23.9
Current period provisions	21.4	39.8
Write-offs charged against allowance	25.1	61.6
Recoveries of amounts previously written off	7.0	20.8
Corporate and Other
Accounts Receivable Allowance For Credit Loss [Line Items]
Accounts Receivable, Allowance for Credit Loss	0.0	0.8	0.8
Current period provisions	0.0	0.0
Write-offs charged against allowance	(0.8)	0.0
Recoveries of amounts previously written off	0.0	0.0
Columbia Operations
Accounts Receivable Allowance For Credit Loss [Line Items]
Accounts Receivable, Allowance for Credit Loss	13.0	10.2	11.1
Current period provisions	15.2	28.3
Write-offs charged against allowance	19.0	49.2
Recoveries of amounts previously written off	6.6	20.0
NIPSCO Operations
Accounts Receivable Allowance For Credit Loss [Line Items]
Accounts Receivable, Allowance for Credit Loss	13.2	11.9	$ 12.0
Current period provisions	6.2	11.5
Write-offs charged against allowance	5.3	12.4
Recoveries of amounts previously written off	$ 0.4	$ 0.8