Related Parties (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
WPL [Member] \| ATC LLC [Member] \| WPL Owed ATC LLC [Member] \| Related Party [Member]
Related Party Transactions [Line Items]
Net amounts owed	$ 10	$ 10

Amount of liabilities classified as other.

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