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Investments in Affiliated Companies (Significant Investments in Affiliates) (Details 1) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Investments in and Advances to Affiliates [Line Items] |  |  |  |
Asset Impairment Charges |  | $ 0.0 | $ 1.5 | $ 48.9
Portfolio proceeds and other |  | 230.6 | 272.8 | 269.6
Investments in and Advances to Affiliates, Schedule of Investments [Abstract] |  |  |  |
Investments in affiliated companies |  | $ 663.3 | 627.0 | 575.1
Rolls Royce Partners Finance [Member] |  |  |  |
Investments in and Advances to Affiliates, Schedule of Investments [Abstract] |  |  |  |
GATX's Percentage Ownership | [1] | 50.00% |  |
RailPulse |  |  |  |
Investments in and Advances to Affiliates, Schedule of Investments [Abstract] |  |  |  |
GATX's Percentage Ownership |  | 10.00% |  |
Rail North America [Member] |  |  |  |
Investments in and Advances to Affiliates [Line Items] |  |  |  |
Asset Impairment Charges |  |  | 0.0 | 0.0
Investments in and Advances to Affiliates, Schedule of Investments [Abstract] |  |  |  |
Investments in affiliated companies |  |  |  | 0.8
Rail North America [Member] | RailPulse |  |  |  |
Investments in and Advances to Affiliates, Schedule of Investments [Abstract] |  |  |  |
Investments in affiliated companies |  | $ 0.2 | 0.2 |
Portfolio Management [Member] |  |  |  |
Investments in and Advances to Affiliates [Line Items] |  |  |  |
Asset Impairment Charges |  | 0.0 | 1.2 | 34.3
Investments in and Advances to Affiliates, Schedule of Investments [Abstract] |  |  |  |
Investments in affiliated companies |  | 663.1 | 626.8 | $ 574.3
Portfolio Management [Member] | Rolls Royce Partners Finance [Member] |  |  |  |
Investments in and Advances to Affiliates, Schedule of Investments [Abstract] |  |  |  |
Investments in affiliated companies |  | $ 663.1 | $ 626.8 |
[1] | (1) Combined investment balances of a group of 50% owned domestic and foreign joint ventures with Rolls-Royce

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