Basis of Presentation (Policies)	9 Months Ended
Sep. 30, 2011
Description of Business and Basis of Presentation [Abstract]
Basis of presentation	The accompanying unaudited consolidated financial statements of GATX Corporation and its subsidiaries have been prepared in accordance with accounting principles generally accepted in the United States of America for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by these accounting principles for complete financial statements. In the opinion of management, all adjustments (which are of a normal recurring nature) considered necessary for a fair presentation have been included. Operating results for the nine months ended September 30, 2011, are not necessarily indicative of the results that may be achieved for the entire year ending December 31, 2011. In particular, ASC’s fleet is generally inactive for a significant portion of the first quarter of each year due to the winter conditions on the Great Lakes. In addition, the timing of asset remarketing income is dependent, in part, on market conditions and, therefore, does not occur evenly from period to period. For further information, refer to the consolidated financial statements and footnotes for the year ended December 31, 2010, as set forth in the Company’s Annual Report on Form 10-K as filed with the Securities and Exchange Commission (“SEC”).
New Accounting Pronouncements	Multiemployer Benefit Plans — In September 2011, the Financial Accounting Standards Board (“FASB”) issued authoritative accounting guidance that requires an entity to provide additional quantitative and qualitative disclosures about its involvement in multiemployer pension and other postretirement benefit plans. The disclosures include the entity’s level of participation in multiemployer benefit plans, the financial health of those plans, and the nature of the entity’s commitments to the plans. The guidance becomes effective for annual periods ending after December 15, 2011, with early adoption permitted. The application of the new guidance will not impact GATX’s financial position, results of operations or cash flows.
Accounting adjustments	In the first quarter of 2010, the Company discovered a clerical error in the preparation of its Consolidated Balance Sheet as of December 31, 2009, and Consolidated Statement of Cash Flows for the quarter and year ended December 31, 2009. The error resulted in a $13.1 million overstatement in each of cash and cash equivalents; accounts payable and accrued expenses; and net cash provided by operating activities. Management has determined that the effect of this error is immaterial and adjusted its Consolidated Balance Sheet and Consolidated Statement of Cash Flows in 2010 to correct this error.
Derivative instruments	GATX recognizes all derivative instruments at fair value and classifies them on the balance sheet as either other assets or other liabilities. Classification of derivative activity in the statements of income and cash flows is generally determined by the nature of the hedged item. Gains and losses on derivatives that are not accounted for as hedges are classified as other operating expenses and the related cash flows are included in cash flows from operating activities. Although GATX does not hold or issue derivative financial instruments for purposes other than hedging, certain derivatives may not qualify for hedge accounting. Changes in the fair value of these derivatives are recognized in earnings immediately.
Variable Interest Entities	GATX evaluates whether an entity is a VIE based on the sufficiency of the entity’s equity and whether the equity holders have the characteristics of a controlling financial interest. To determine if it is the primary beneficiary of a VIE, GATX assesses whether it has the power to direct the activities that most significantly impact the economic performance of the VIE and the obligation to absorb losses or the right to receive benefits that may be significant to the VIE. These determinations are both qualitative and quantitative in nature and require certain judgments and assumptions about the VIE’s forecasted financial performance and the volatility inherent in those forecasted results. GATX evaluates new investments for VIE determination and regularly reviews all existing entities for any events that may result in an entity becoming a VIE or GATX becoming the primary beneficiary of an existing VIE. GATX is the primary beneficiary of a consolidated VIE related to a structured lease financing for a portfolio of railcars because it has the power to direct the significant activities of the VIE through its ownership of the equity interests in the transaction.
Earnings Per Share	Basic earnings per share were computed by dividing net income available to common shareholders by the weighted average number of shares of common stock outstanding during each period. Shares issued or reacquired during the period, if applicable, were weighted for the portion of the period that they were outstanding. Diluted earnings per share give effect to potentially dilutive securities, including convertible preferred stock, employee stock options/SARs, restricted stock and convertible debt.

The entire disclosure for reporting accounting changes and error corrections. It includes the conveyance of information necessary for a user of the Company's financial information to understand all aspects and required disclosure information concerning all changes and error corrections reported in the Company's financial statements for the period.

Disclosure of accounting policy for consolidation to describe the significant judgments and assumptions made in determining whether a variable interest held by the entity requires the variable interest entity to be consolidated and (or) disclose information about its involvement with the variable interest entity; the methodology used by the entity for determining whether or not it is the primary beneficiary of the variable interest entity; and the significant factors considered and judgments made in determining that the power to direct the activities that significantly impact the economic performance of the variable interest entity are shared (as defined).

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- Definition

Disclosure of accounting policy for its derivative instruments and hedging activities. Disclosure may include: (1) Each method used to account for derivative financial instruments and derivative commodity instruments ("derivatives"); (2) the types of derivatives accounted for under each method; (3) the criteria required to be met for each accounting method used, including a discussion of the criteria required to be met for hedge or deferral accounting and accrual or settlement accounting (for example, whether and how risk reduction, correlation, designation, and effectiveness tests are applied); (4) the accounting method used if the criteria specified for hedge accounting are not met; (5) the method used to account for termination of derivatives designated as hedges or derivatives used to affect directly or indirectly the terms, fair values, or cash flows of a designated item; (6) the method used to account for derivatives when the designated item matures, is sold, is extinguished, or is terminated. In addition, the method used to account for derivatives designated to an anticipated transaction, when the anticipated transaction is no longer likely to occur; and (7) where and when derivatives, and their related gains or losses are reported in the statement of financial position, cash flows, and results of operations and (8) an accounting policy decision to offset fair value amounts with counterparties. An entity may also describe its embedded derivatives, and the method(s) used to determine the fair values of derivatives and any significant assumptions used in such valuations.

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Disclosure of accounting policy for computing basic and diluted earnings or loss per share for each class of common stock and participating security. Addresses all significant policy factors, including any antidilutive items that have been excluded from the computation and takes into account stock dividends, splits and reverse splits that occur after the balance sheet date of the latest reporting period but before the issuance of the financial statements.

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