Investments in Affiliated Companies (Investments and Distributions) (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013 Vessel	Dec. 31, 2012
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Net Sales Proceeds	$ 19.4	$ 55.6	$ 12.5
Number of vessels received		5gmt_Numberofvesselsreceived
Payments to Acquire Equity Method Investments	15.3	101.3	29.7
Proceeds from Equity Method Investment, Dividends or Distributions	54.2	47.0	53.2
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	20.0	0	14.9
Rail International [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	0	0
Portfolio Management [Member]
Schedule of Equity Method Investments [Line Items]
Payments to Acquire Equity Method Investments	15.3	101.3	29.7
Proceeds from Equity Method Investment, Dividends or Distributions	34.2	47.0	38.3
Enerven Compression Llc [Member]
Schedule of Equity Method Investments [Line Items]
Reversal of Impairment Loss		$ 1.1

Number of vessels received

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Amount of an impairment reversed related as a result of higher-than-expected proceeds from the sale of an asset.

+ References

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- Definition

This item represents disclosure of the amount of dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporation; these investments are accounted for under the equity method of accounting. This element excludes distributions that constitute a return of investment, which are classified as investing activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 13

-Article 5

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- Definition

This item represents the amount of net sales proceeds received on disposal of an equity method investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 35

-Paragraph 35

-URI http://asc.fasb.org/extlink&oid=35687916&loc=d3e32847-111569

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- Definition

The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 13

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585

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