Investments in Affiliated Companies (Investments and Distributions) (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013 Vessel	Dec. 31, 2012	Dec. 31, 2011
Schedule of Equity Method Investments [Line Items]
Number of vessels received	5
Payments to Acquire Equity Method Investments	$ 101.3	$ 29.7	$ 116.2
Proceeds from Equity Method Investment, Dividends or Distributions	77.0	65.7	35.3
Rail North America [Member]
Schedule of Equity Method Investments [Line Items]
Payments to Acquire Equity Method Investments	0	0	0
Proceeds from Equity Method Investment, Dividends or Distributions	0	14.9	8.0
Rail International [Member]
Schedule of Equity Method Investments [Line Items]
Payments to Acquire Equity Method Investments	0	0	2.8
Proceeds from Equity Method Investment, Dividends or Distributions	0	0	0
Portfolio Management [Member]
Schedule of Equity Method Investments [Line Items]
Payments to Acquire Equity Method Investments	101.3	29.7	113.4
Proceeds from Equity Method Investment, Dividends or Distributions	77.0	50.8	27.3
Enerven Compression Llc [Member]
Schedule of Equity Method Investments [Line Items]
gmt_ReversalofImpairmentLoss	$ 1.1