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Shareholders' Equity (Details) (USD $) Share data in Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2014
Class of Stock [Line Items] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 3 years |  |  |
Stock Repurchase Program, Authorized Amount |  | $ 200,000,000 us-gaap_StockRepurchaseProgramAuthorizedAmount1 |  | $ 250,000,000 us-gaap_StockRepurchaseProgramAuthorizedAmount1
Common stock reserved for conversion and incentive plans |  |  |  |
Common stock reserved for conversion and incentive plans | 6.0 us-gaap_CommonStockCapitalSharesReservedForFutureIssuance |  |  |
Stock Repurchased During Period, Shares | 1.9 us-gaap_StockRepurchasedDuringPeriodShares | 1.4 us-gaap_StockRepurchasedDuringPeriodShares | 0 us-gaap_StockRepurchasedDuringPeriodShares |
Share-based compensation award plans [Member] |  |  |  |
Common stock reserved for conversion and incentive plans |  |  |  |
Common stock reserved for conversion and incentive plans | 2.5 us-gaap_CommonStockCapitalSharesReservedForFutureIssuance / us-gaap_StatementClassOfStockAxis = gmt_SharebasedcompensationawardplansMember |  |  |
GATX Corporation 2004 Equity Incentive Compensation Plan [Member] |  |  |  |
Common stock reserved for conversion and incentive plans |  |  |  |
Common stock reserved for conversion and incentive plans | 3.5 us-gaap_CommonStockCapitalSharesReservedForFutureIssuance / us-gaap_StatementClassOfStockAxis = us-gaap_StockCompensationPlanMember |  |  |
Restricted Stock and Performance Share Awards [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 3 years |  |  |

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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5 + Details
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