Investments in Affiliated Companies (Details 1) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Investment	$ 513.8	$ 486.1	$ 452.2
Rolls Royce Partners Finance [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Investment	179.6
GATX's Percentage Ownership	50.00%
AAE Cargo AG [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Investment	85.4
GATX's Percentage Ownership	37.50%
Cardinal Marine Investments LLC [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Investment	48.3
GATX's Percentage Ownership	50.00%
Enerven Compression LLC [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Investment	49.3
GATX's Percentage Ownership	45.60%
Adler Funding LLC [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Investment	$ 35.2
GATX's Percentage Ownership	12.50%