Fair Value Disclosure (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Interest Expense [Member] | Fair Value Hedging [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Interest expense	$ 2.1	[1]	$ 1.1	[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	1.2		1.2
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)	1.8		(0.3)
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	2.4		0.4
Other Expense [Member] | Non-designated [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense	$ 0.1		$ 0.5

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