Investments in Affiliated Companies (Significant Investments in Affiliates) (Details 1) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013 Vessel		Dec. 31, 2011	Dec. 31, 2012
Investments in and Advances to Affiliates [Line Items]
Advances to Affiliate	$ 24.2			$ 51.3
Number of vessels received	5
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	354.3		513.8	502.0
Fair Value of Assets Acquired	22.9		88.8
Rolls Royce Partners Finance [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	248.2	[1]
GATX's Percentage Ownership	50.00%	[1]
Cardinal Marine Investments Llc [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	48.8
GATX's Percentage Ownership	50.00%
Adler Funding Llc [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	24.7
GATX's Percentage Ownership	12.50%
Other Affiliates [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	32.6
RRPF Joint Ventures [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Number of joint venture investments	14
Singco/Somagas [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Fair Value of Assets Acquired	$ 151.8
Aae Cargo Ag [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Percentage Ownership	37.50%

[1] (1) Combined investment balances of fourteen separate joint ventures (collectively, the "RRPF affiliates")