Sale price for an investment in an affiliate sold during the period

No definition available.

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Balance Type:	credit
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- Definition

Number of Business Activities

+ References

No definition available.

+ Details

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- Definition

Number of Joint Venture Investments

+ References

No definition available.

+ Details

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Namespace Prefix:	gmt_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Operating Leases, Lease Term

+ References

No definition available.

+ Details

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Data Type:	xbrli:durationItemType
Balance Type:	na
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- Definition

Portfolio proceeds.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Long-Term advances receivable from a party that is affiliated with the reporting entity by means of direct or indirect ownership. This does not include advances to clients.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 946

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.6-04.2)

-URI http://asc.fasb.org/extlink&oid=6488278&loc=d3e603758-122996

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section 45

-Paragraph 4

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6812-107765

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 2

-Article 6

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571

+ Details

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Balance Type:	na
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- Definition

The difference between the carrying value and the sale price of an investment. A gain would be recognized when the sale price of the investment is greater than the carrying value of the investment. This element refers to the Gain included in earnings and not to the cash proceeds of the sale.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

+ 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 9

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=35755396&loc=d3e33749-111570

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.12)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 11

-Article 7

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The value of an asset or business acquired in a noncash (or part noncash) acquisition. Noncash is defined as information about all investing and financing activities of an enterprise during a period that affect recognized assets or liabilities but that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash inflow associated with the amount received from the sale of a business segment or subsidiary or sale of an entity that is related to it but not strictly controlled during the period (for example, an unconsolidated subsidiary, affiliate, joint venture or equity method investment).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 12

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3179-108585

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Details

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Investments in Affiliated Companies (Textual) (Details) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2014 RRPF Joint Ventures [Member] USD ($) Business_Activity Joint_Venture	Dec. 31, 2013 RRPF Joint Ventures [Member] USD ($)	Dec. 31, 2012 RRPF Joint Ventures [Member] USD ($)	Dec. 31, 2014 RRPF Joint Ventures [Member] Minimum [Member]	Dec. 31, 2014 RRPF Joint Ventures [Member] Maximum [Member]	Dec. 31, 2014 RRPF Joint Ventures [Member] Aircraft Engines [Member]	Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member] Fair Value, Inputs, Level 3 [Member] USD ($)	Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member] Accumulated Other-than-Temporary Impairment [Member] Fair Value, Inputs, Level 3 [Member] Equity Method Investments [Member] USD ($)	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 (€)	Dec. 31, 2013 Singco/Somagas [Member] USD ($)	Dec. 31, 2013 Singco/Somagas [Member] USD ($)	Dec. 31, 2014 Aae Cargo Ag [Member] USD ($)	Dec. 31, 2013 Aae Cargo Ag [Member] USD ($)	Dec. 31, 2012 Aae Cargo Ag [Member] USD ($)
Schedule of Equity Method Investments [Line Items]
Noncash or Part Noncash Acquisition, Value of Assets Acquired	$ 1.1	$ 174.7															$ 101.3
Loans to affiliated companies	16.3us-gaap_AdvancesToAffiliate	24.2us-gaap_AdvancesToAffiliate
Asset Impairment Charges	1.3us-gaap_AssetImpairmentCharges	5.9us-gaap_AssetImpairmentCharges	5.0us-gaap_AssetImpairmentCharges							14.8	14.8
GATX ownership percentage in joint venture				50.00%
Number of joint venture investments				14
Number of business activities				2
Estimated useful lives of depreciable assets									25 years
Lease term							7 years	10 years
Share of affiliates' earnings (net of tax)	49.5	75.8	19.6	42.2	46.5	36.7
Noncontrolling Interest, Ownership Percentage by Parent														37.50%	37.50%
Gain on Sale of Investments		2.5us-gaap_GainOnSaleOfInvestments												9.3
gmt_SalePrice														114.1
Proceeds from Divestiture of Businesses and Interests in Affiliates																23.0
Portfolio proceeds	$ 264.0gmt_PortfolioProceeds	$ 385.3gmt_PortfolioProceeds	$ 288.9gmt_PortfolioProceeds									$ 77.0	€ 68.2	$ 91.1	€ 67.5			$ 0	$ 91.1	$ 0