Investments in Affiliated Companies (Significant Investments in Affiliates) (Details 1) In Millions, unless otherwise specified	12 Months Ended									12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($) Vessel	Dec. 31, 2012 USD ($)	Dec. 31, 2014 Rolls Royce Partners Finance [Member] USD ($)		Dec. 31, 2014 Cardinal Marine Investments Llc [Member] USD ($)	Dec. 31, 2014 Adler Funding Llc [Member] USD ($)	Dec. 31, 2014 Other Affiliates [Member] USD ($)	Dec. 31, 2014 RRPF Joint Ventures [Member] Joint_Venture	Dec. 31, 2013 Singco/Somagas [Member] USD ($)	Dec. 31, 2014 Intermodal Investment Fund V and VII [Member] USD ($)	Dec. 31, 2014 Aae Cargo Ag [Member] USD ($)	Dec. 31, 2014 Aae Cargo Ag [Member] EUR (€)	Dec. 31, 2013 Aae Cargo Ag [Member] USD ($)	Dec. 31, 2013 Aae Cargo Ag [Member] EUR (€)
Investments in and Advances to Affiliates [Line Items]
Advances to Affiliate	$ 16.3	$ 24.2
Equity Method Investment, Net Sales Proceeds	19.4	55.6	12.5								18.3
Portfolio proceeds and other	264.0	385.3	288.9									77.0	68.2	91.1	67.5
Number of vessels received		5
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	357.7	354.3	502.0	293.6	[1]	45.1	16.9	2.1
GATX's Percentage Ownership				50.00%	[1]	50.00%	12.50%							37.50%	37.50%
Number of joint venture investments									14
Fair Value of Assets Acquired	$ 1.1	$ 22.9	$ 0							$ 151.8

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