Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Effective Income Tax Rate Reconciliation, Percent	41.00%	32.70%	41.20%
GATX income taxes on sale of AAE	$ 0.0	$ 0.0	$ 23.2
Deferred Tax Liabilities
Book/tax basis difference due to depreciation	1,058.1	963.8
Investments in affiliated companies	72.9	99.7
Lease accounting (other than leveraged)	7.3	10.5
Other	0.0	0.1
Total deferred tax liabilities	1,138.3	1,074.1
Deferred Tax Assets
Alternative minimum tax credit	14.8	9.2
Federal net operating loss	13.2	41.8
Foreign tax credit	5.8	3.6
Valuation on foreign tax credit	(5.8)	(3.6)
State net operating loss	27.7	28.7
Valuation on state net operating loss	(12.6)	(11.3)
Foreign net operating loss	3.6	5.8
Deferred Tax Assets, Valuation Allowance	(0.3)	(0.2)
Accruals not currently deductible for tax purposes	22.2	18.7
Allowance for losses	3.2	1.7
Pension and post-retirement benefits	35.8	32.6
Other	12.4	9.8
Total deferred tax assets	120.0	136.8
Net deferred tax liabilities	1,018.3	937.3
Gross liability for unrecognized tax benefits
Beginning balance	5.6	5.7
Reductions due to expiration of the applicable statute of limitations	0.0	0.4
Reductions due to settlement of audit issues	(0.1)	(0.3)
Ending balance	5.7	5.6	5.7
Effective Income Tax Rate Reconciliation, Tax Credit, Foreign, Amount	0.0	0.0	3.9
Expiration of the applicable statute of limitations		(0.2)
Effective Income Tax Rate Reconciliation, Repatriation of Foreign Earnings, Amount	$ 0.9	$ 0.0	$ 0.0