v3.26.1
Fair Value of Financial Instruments (Details) - EBP 002 - USD ($)
Dec. 31, 2025
Dec. 31, 2024
EBP, Investment, Fair Value and NAV [Line Items]    
Plan interest in the GATX Corporation Master Trust $ 263,756,200 $ 238,226,571
Level 1    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan interest in the GATX Corporation Master Trust 287,555,524 260,354,885
Level 1 | Mutual funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan interest in the GATX Corporation Master Trust 114,161,653 98,986,627
Level 1 | GATX common stock fund    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan interest in the GATX Corporation Master Trust 31,944,636 30,869,976
Level 1 | Collective trusts    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan interest in the GATX Corporation Master Trust 141,449,235 130,498,282
Level 2    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan interest in the GATX Corporation Master Trust 0 0
Level 3    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan interest in the GATX Corporation Master Trust $ 0 $ 0