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v3.19.1

Leases Lessee, Maturity Table (Tables) | 3 Months Ended
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Mar. 31, 2019
Lessee Leases Future Minimum Payments [Abstract] |
Lessee, Leases, Future Minimum Payments [Table Text Block] |
| March 31, 2019
| Operating Leases |  | Finance Leases |  | Total
2019 (1) | $ | 34.8 |  |  | $ | 11.1 |  |  | $ | 45.9 |
2020 | 67.7 |  |  | — |  |  | 67.7 |
2021 | 66.1 |  |  | — |  |  | 66.1 |
2022 | 57.2 |  |  | — |  |  | 57.2 |
2023 | 54.4 |  |  | — |  |  | 54.4 |
Years thereafter | 268.2 |  |  | — |  |  | 268.2 |
Total undiscounted lease payments | $ | 548.4 |  |  | $ | 11.1 |  |  | $ | 559.5 |
Less: amounts representing interest | (92.1 | ) |  | (0.1 | ) |  | (92.2 | )
Total discounted lease liabilities | $ | 456.3 |  |  | $ | 11.0 |  |  | $ | 467.3 |
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(1) | For the remainder of the year.

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- Definition Lessee Leases Future Minimum Payments [Abstract] + References No definition available. + Details
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Namespace Prefix: | gmt_
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- Definition [Table Text Block] for Lessee, Leases, Future Minimum Payments [Table] + References No definition available. + Details
Name: | gmt_LesseeLeasesFutureMinimumPaymentsTableTextBlock
Namespace Prefix: | gmt_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration