Investments in Affiliated Companies (Significant Investments in Affiliates) (Details 1) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	$ 502.0		$ 513.8	$ 486.1
Rolls Royce Partners Finance [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	208.7	[1]
GATX's Percentage Ownership	50.00%	[1]
Aae Cargo Ag [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	74.7
GATX's Percentage Ownership	37.50%
Cardinal Marine Investments Llc [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	52.6
GATX's Percentage Ownership	50.00%
Singco Gas Pte Ltd [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	40.5
GATX's Percentage Ownership	50.00%
Enerven Compression Llc [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	32.9
GATX's Percentage Ownership	45.60%
Adler Funding Llc [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	26.7
GATX's Percentage Ownership	12.50%
Southern Capital Corp [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	20.2
GATX's Percentage Ownership	50.00%
Intermodal Investment Fund V [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	18.1
GATX's Percentage Ownership	50.00%
Somargas Ii Private Limited [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	15.4
GATX's Percentage Ownership	35.00%
Other Affiliates [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	$ 12.2
RRPF Joint Ventures [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Number of joint venture investments	14

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