Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Effective Income Tax Rate Reconciliation, Percent	32.70%	41.20%	18.20%
GATX income taxes on sale of AAE	$ 0	$ 23.2	$ 0
Deferred Tax Liabilities
Book/tax basis difference due to depreciation	963.8	852.0
Investments in affiliated companies	99.7	93.7
Lease accounting (other than leveraged)	10.5	24.4
Other	0.1	0
Total deferred tax liabilities	1,074.1	970.1
Deferred Tax Assets
Alternative minimum tax credit	9.2	8.4
Federal net operating loss	41.8	8.9
Foreign tax credit	3.6	3.6
Valuation on foreign tax credit	(3.6)	(3.6)
State net operating loss	28.7	27.1
Valuation on state net operating loss	(11.3)	(11.2)
Foreign net operating loss	5.8	8.5
Deferred Tax Assets, Valuation Allowance	(0.2)	(0.2)
Accruals not currently deductible for tax purposes	18.7	14.1
Allowance for losses	1.7	2.2
Pension and post-retirement benefits	32.6	13.3
Other	9.8	7.6
Total deferred tax assets	136.8	78.7
Net deferred tax liabilities	937.3	891.4
Gross liability for unrecognized tax benefits
Beginning balance	5.7	4.7
Reductions due to expiration of the applicable statute of limitations	0.4	0
Reductions due to settlement of audit issues	(0.3)	(1.0)
Ending balance	5.6	5.7	4.7
Effective Income Tax Rate Reconciliation, Tax Credit, Foreign, Amount	0	3.9	13.7
Expiration of the applicable statute of limitations	0.2	(0.6)	15.5
Effective Income Tax Rate Reconciliation, Repatriation of Foreign Earnings, Amount	$ 0	$ 0	$ 6.3