Subsequent Events	12 Months Ended
Dec. 31, 2020
Subsequent Event [Line Items]
Unusual or Infrequent Items, or Both, Disclosure	Coronavirus Impacts On March 11, 2020, the World Health Organization declared the Coronavirus Disease 2019 (“COVID-19”) a pandemic and on March 13, 2020, the United States declared a national emergency related to COVID-19. Our consolidated financial statements reflect estimates and assumptions at the date of the consolidated financial statements and reported amounts of revenue and expenses during the reporting periods presented. We considered the impact of COVID-19 on our operations and the assumptions and estimates used. While COVID-19 did have a negative impact on operating conditions in 2020, we determined the impact to our assumptions and estimates was not significant. However, we expect COVID-19 will continue to have an adverse impact on our operating and financial results in future periods, the magnitude and duration of which cannot be determined at this time. On March 27, 2020, the U.S. government enacted the Coronavirus Aid, Relief, and Economic Security Act (“CARES Act”), which includes modifications to the interest expense limitation threshold and net operating loss carryback period and utilization limitation, the acceleration of payments for alternative minimum tax credit refunds, and the deferral of employer payroll tax payments. The CARES Act did not have a material impact on our consolidated financial statements.
Mergers, Acquisitions and Dispositions Disclosures	Business Combinations On December 29, 2020, GATX acquired Trifleet Leasing Holding B.V. ("Trifleet"), the fourth largest tank container lessor in the world, for approximately €165 million ($203.2 million) in cash. Transaction costs associated with this acquisition were approximately $2.7 million. Headquartered in the Netherlands with offices worldwide, Trifleet owns and manages a fleet of over 19,000 tank containers leased to a diverse customer base in the chemical, industrial gas, energy, food, cryogenic and pharmaceutical industries, as well as to tank container operators. We have allocated $146.2 million and $57.0 million to tangible net assets and goodwill in the preliminary purchase accounting for the acquisition. The initial allocation of the purchase price is incomplete with respect to certain assets and liabilities acquired. The purchase price allocation will be finalized during the measurement period, which will not exceed 12 months from the acquisition date. The acquisition was not significant in relation to our financial results and, therefore, pro-forma financial information has not been presented.
Subsequent Events	Subsequent EventsSubsequent to December 31, 2020, we invested approximately $120 million at Portfolio Management for the acquisition of Rolls-Royce aircraft spare engines that are on long-term leases to strong airline customers. The investment was funded with proceeds from a draw on our delayed draw term loan in January 2021. RRPF will continue to invest at the joint venture level while also managing these direct investments for GATX. The assets acquired, and the related leasing activities and financial results, will be reported in the Portfolio Management segment.

Subsequent Events

12 Months Ended

Dec. 31, 2020

Unusual or Infrequent Items, or Both, Disclosure

Coronavirus Impacts On March 11, 2020, the World Health Organization declared the Coronavirus Disease 2019 (“COVID-19”) a pandemic and on March 13, 2020, the United States declared a national emergency related to COVID-19. Our consolidated financial statements reflect estimates and assumptions at the date of the consolidated financial statements and reported amounts of revenue and expenses during the reporting periods presented. We considered the impact of COVID-19 on our operations and the assumptions and estimates used. While COVID-19 did have a negative impact on operating conditions in 2020, we determined the impact to our assumptions and estimates was not significant. However, we expect COVID-19 will continue to have an adverse impact on our operating and financial results in future periods, the magnitude and duration of which cannot be determined at this time. On March 27, 2020, the U.S. government enacted the Coronavirus Aid, Relief, and Economic Security Act (“CARES Act”), which includes modifications to the interest expense limitation threshold and net operating loss carryback period and utilization limitation, the acceleration of payments for alternative minimum tax credit refunds, and the deferral of employer payroll tax payments. The CARES Act did not have a material impact on our consolidated financial statements.

Mergers, Acquisitions and Dispositions Disclosures

Business Combinations

On December 29, 2020, GATX acquired Trifleet Leasing Holding B.V. ("Trifleet"), the fourth largest tank container lessor in the world, for approximately €165 million ($203.2 million) in cash. Transaction costs associated with this acquisition were approximately $2.7 million. Headquartered in the Netherlands with offices worldwide, Trifleet owns and manages a fleet of over 19,000 tank containers leased to a diverse customer base in the chemical, industrial gas, energy, food, cryogenic and pharmaceutical industries, as well as to tank container operators.

We have allocated $146.2 million and $57.0 million to tangible net assets and goodwill in the preliminary purchase accounting for the acquisition. The initial allocation of the purchase price is incomplete with respect to certain assets and liabilities acquired. The purchase price allocation will be finalized during the measurement period, which will not exceed 12 months from the acquisition date. The acquisition was not significant in relation to our financial results and, therefore, pro-forma financial information has not been presented.

Subsequent Events

Subsequent EventsSubsequent to December 31, 2020, we invested approximately $120 million at Portfolio Management for the acquisition of Rolls-Royce aircraft spare engines that are on long-term leases to strong airline customers. The investment was funded with proceeds from a draw on our delayed draw term loan in January 2021. RRPF will continue to invest at the joint venture level while also managing these direct investments for GATX. The assets acquired, and the related leasing activities and financial results, will be reported in the Portfolio Management segment.