| Document and Entity Information Statement | Jan. 28, 2021 |
| Document Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Jan. 28, 2021 |
| Entity Registrant Name | GATX Corporation |
| Entity Incorporation, State or 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 |
| 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 |
| Amendment Flag | false |
| Entity Central Index Key | 0000040211 |
| CHICAGO STOCK EXCHANGE, INC [Member] | |
| Document Information [Line Items] | |
| Security Exchange Name | CHX |
| Common Stock [Member] | NEW YORK STOCK EXCHANGE, INC. [Member] | |
| Document Information [Line Items] | |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | GATX |
| Security Exchange Name | NYSE |
| Senior Notes [Member] | NEW YORK STOCK EXCHANGE, INC. [Member] | |
| Document Information [Line Items] | |
| Title of 12(b) Security | 5.625% Senior Notes due 2066 |
| Trading Symbol | GMTA |
| Security Exchange Name | NYSE |
