Fair Value Disclosure - Derivative Instruments (Details) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018 USD ($) Instrument	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Instrument	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Instrument
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Derivative Asset, Notional Amount		$ 500.0		$ 500.0		$ 550.0
Fair Value Hedging [Member]
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Derivative, Number of Instruments Held | Instrument		9		9		10
Cash Flow Hedges [Member]
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Derivative, Number of Instruments Held | Instrument		11		11		5
Interest Expense [Member] | Fair Value Hedging [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Interest expense	[1]	$ 1.8	$ (1.2)	$ 7.6	$ 0.9
Interest Expense [Member] | Cash Flow Hedges [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Effective portion reclassified from accumulated other comprehensive loss		1.1	1.7	2.2	3.4
Other Comprehensive Income (Loss) [Member] | Cash Flow Hedges [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other comprehensive loss (effective portion)		17.2	(18.6)	5.7	(23.7)
Other Nonoperating Income (Expense) [Member] | Cash Flow Hedges [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Effective portion reclassified from accumulated other comprehensive loss		(16.3)	19.6	(8.1)	23.7
Other Expense [Member] | Non-designated [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense		$ (5.0)	$ 1.3	$ (5.7)	$ 6.1

[1] (1) The fair value adjustments related to the underlying debt equally offset the amounts recognized in interest expense.