Investments in Affiliated Companies (Textual) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended										3 Months Ended		12 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012 Vessel	Dec. 31, 2011 Vessel	Dec. 31, 2013 RRPF Joint Ventures [Member] Business_Activity Joint_Venture	Dec. 31, 2012 RRPF Joint Ventures [Member]	Dec. 31, 2011 RRPF Joint Ventures [Member]	Dec. 31, 2013 RRPF Joint Ventures [Member] Minimum [Member]	Dec. 31, 2013 RRPF Joint Ventures [Member] Maximum [Member]	Dec. 31, 2013 RRPF Joint Ventures [Member] Aircraft Engines [Member]	Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member] Investments Impairment Charge [Member] Fair Value, Inputs, Level 3 [Member] Equity Method Investments [Member]	Dec. 31, 2013 Aae Cargo Ag [Member]	Mar. 31, 2013 Aae Cargo Ag [Member]	Dec. 31, 2013 Aae Cargo Ag [Member]	Dec. 31, 2013 Singco/Somagas [Member]	Dec. 31, 2013 Singco/Somagas [Member]	Dec. 31, 2013 Aae Cargo Ag [Member]	Dec. 31, 2012 Aae Cargo Ag [Member]	Dec. 31, 2011 Aae Cargo Ag [Member]
Schedule of Equity Method Investments [Line Items]
Noncash or Part Noncash Acquisition, Value of Assets Acquired	$ 174.7	$ 0													$ 101.3
Loans to affiliated companies	24.2	51.3
Asset Impairment Charges	5.9	5.0	6.8							14.8
Impairment loss			5.2
GATX ownership percentage in joint venture				50.00%
Contribution for payment of outstanding debt on dissolution			62.1
Number of vessels containing liquidating distributions		6	6
Fair value of vessels received on dissolution			88.8
Number of joint venture investments				14
Number of business activities				2
Estimated useful lives of depreciable assets									25 years
Lease term							7 years	10 years
Share of affiliates' earnings (net of tax)	75.8	19.6	32.4	46.5	36.7	30.8
Noncontrolling Interest, Ownership Percentage by Parent											37.50%		37.50%
Gain on Sale of Investments	2.5												9.3
gmt_SalePrice													114.1
Proceeds from Divestiture of Businesses and Interests in Affiliates														23.0
Portfolio proceeds	$ 385.3	$ 288.9	$ 154.1								$ 91.1	$ 91.1				$ 91.1	$ 0	$ 0