Fair Value Disclosure - Derivative Instruments (Details) $ in Millions		3 Months Ended				9 Months Ended		12 Months Ended
		Sep. 30, 2018 USD ($) Instrument	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Instrument	Sep. 30, 2017 USD ($)	Dec. 31, 2015 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
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other comprehensive loss (effective portion)		$ 2.8	$ 18.0	$ (11.5)
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		8				8			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]	$ 1.0			$ 0.6	$ 8.6	$ 1.5
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	3.3	5.1
Operating 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		0.1			0.0	0.1		$ 0.0
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)		2.7			(11.1)	8.4	(34.8)
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		(2.2)			10.1	(10.3)	33.8
Other Expense [Member] | Non-designated [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense		$ 3.0			$ (2.0)	$ (2.7)	$ 4.1

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