Fair Value Disclosure (Details 2) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016		Jun. 30, 2015
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	$ (1.4)	$ (5.9)	[1]	$ (0.9)	[1]
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	1.7	1.4	3.4		2.7
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)	(0.8)	10.9	(24.0)		2.5
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	0.1	0.4		0.2
Other Expense [Member] | Cash Flow Hedges [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense	(6.7)	12.6	10.3		4.2
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	(3.6)	(1.6)	(2.3)		(5.8)
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	$ 10.2	$ 12.6	$ 6.8		$ 2.7

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