| Document and Entity Information | Apr. 15, 2021 |
| Document And Entity Information [Line Items] | |
| Entity Registrant Name | GATX CORP |
| Amendment Flag | false |
| Entity Central Index Key | 0000040211 |
| Document Type | 8-K |
| Document Period End Date | Apr. 15, 2021 |
| Entity Incorporation State Country Code | NY |
| Entity File Number | 1-2328 |
| Entity Tax Identification Number | 36-1124040 |
| Entity Address, Address Line One | 233 South Wacker Drive |
| Entity Address, City or Town | Chicago |
| Entity Address, State or Province | IL |
| Entity Address, Postal Zip Code | 60606-7147 |
| City Area Code | (312) |
| Local Phone Number | 621-6200 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Common Stock |
| Trading Symbol | GATX |
| Security Exchange Name | NYSE |
| M 0 [Member] | |
| Document And Entity Information [Line Items] | |
| Security Exchange Name | CHX |
| Senior Notes [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 5.625% Senior Notes due 2066 |
| Trading Symbol | GMTA |
| Security Exchange Name | NYSE |
