Fair Value Disclosure (Details 2) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Interest Expense [Member] | Fair Value Hedging [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Interest expense	[1]	$ (4.4)	$ 3.2	$ (5.3)	$ 4.8
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		1.4	1.3	4.1	3.7
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)		(0.5)	0.5	2.0	3.7
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	0.3	0.3	0.8
Other Expense [Member] | Cash Flow Hedges [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense		0.8	0.0	5.0	2.2
Not Designated as Hedging Instrument [Member] | Other Expense [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense		(0.6)	$ (7.0)	(6.4)	$ (6.0)
Not Designated as Hedging Instrument [Member] | Other Expense [Member] | Aae Cargo Ag [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense		5.1
Foreign Currency Gain (Loss) [Member] | Other Expense [Member] | Cash Flow Hedges [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense		$ 0.8		$ 3.5

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