Investments in Affiliated Companies (Summarized Financial Data-RRPF) (Details 6) $ in Millions		12 Months Ended
		Dec. 31, 2025 USD ($) AircraftEngines	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Schedule of Equity Method Investments [Line Items]
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment		$ 4,251.7	$ 3,880.9
Income Statement [Abstract]
Total Revenues		1,740.4	1,585.5	$ 1,410.9
Depreciation expense		(451.7)	(420.9)	(391.5)
Interest expense, net		(391.5)	(341.0)	(263.4)
Income Taxes		(63.1)	(60.0)	(58.7)
Net income attributable to GATX		333.3	284.2	259.2
Assets [Abstract]
Total assets		17,999.5	12,296.5	11,326.0
Accounts payable and accrued expenses		318.4	217.1
Liabilities and Equity [Abstract]
Other liabilities		162.1	107.5
Shareholders’ equity		884.6	0.0
Total Liabilities and Shareholders’ Equity		17,999.5	12,296.5
Payment for Management Fee		5.6	4.1	2.7
Payment for Management Fee		5.6	4.1	2.7
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Income Statement [Abstract]
Total Revenues		643.4	514.0	487.2
Gains on sales of assets		134.6	75.2	91.7
Net income attributable to GATX		238.4	164.0	150.6
Assets [Abstract]
Current assets		521.5	610.5
Noncurrent assets		5,897.6	4,709.1
Total assets		6,419.1	5,319.6
Liabilities and Equity [Abstract]
Current liabilities		883.4	618.4
Liabilities, Noncurrent		4,102.4	3,405.1
Total Liabilities and Shareholders’ Equity		6,419.1	5,319.6
RRPF Joint Ventures [Member]
Income Statement [Abstract]
Depreciation expense		(224.4)	(206.2)	(223.7)
Interest expense, net		(172.0)	(127.3)	(135.9)
Other expenses		(64.4)	(37.7)	(19.3)
Gains on sales of assets		134.5	75.5	92.6
Income (Loss) Attributable to Parent, before Tax		314.3	216.6	197.5
Income Taxes		(74.4)	(49.4)	(42.5)
Net income attributable to GATX		239.9	167.2	155.0
Assets [Abstract]
Current assets		519.3	607.9
Noncurrent assets	[1]	5,897.6	4,709.1
Total assets		6,416.9	5,317.0
Accounts payable and accrued expenses		235.3	239.6
Liabilities and Equity [Abstract]
Current liabilities		645.1	378.2
Liabilities, Noncurrent		3,189.4	2,673.7
Other liabilities		913.0	731.3
Shareholders’ equity		1,434.1	1,294.2
Total Liabilities and Shareholders’ Equity		6,416.9	5,317.0
Collateral of long-term debt		4,620.4
Third Parties [Member] | RRPF Joint Ventures [Member]
Income Statement [Abstract]
Total Revenues		401.4	268.1	224.8
Rolls Royce [Member] | RRPF Joint Ventures [Member]
Income Statement [Abstract]
Total Revenues		215.1	225.3	253.0
Other revenue		$ 24.1	$ 18.9	$ 6.0
RRPF Joint Ventures [Member]
Liabilities and Equity [Abstract]
Number of Aircraft Engines Under Leasing Arrangement | AircraftEngines		456

[1] (1) $4,620.4 million of operating assets were pledged as collateral for long-term debt obligations at December 31, 2025.