XML 44 R30.htm IDEA: XBRL DOCUMENT
v3.23.2

Leases - Maturities of Lease Liabilities (Details) - USD ($) $ in Millions | Jun. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Operating Leases |  |
Total discounted lease liabilities | $ 241.1 | $ 257.9
Total |  |
Total discounted lease liabilities | $ 241.1 | $ 257.9

X
- Definition Lease Liabilities, Payments, Due [Abstract] + References No definition available. + Details
Name: | gmt_LeaseLiabilitiesPaymentsDueAbstract
Namespace Prefix: | gmt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Lease, Liability + References No definition available. + Details
Name: | gmt_LeaseLiability
Namespace Prefix: | gmt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_OperatingLeaseLiabilitiesPaymentsDueAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Present value of lessee's discounted obligation for lease payments from operating lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479041/842-20-45-1 + Details
Name: | us-gaap_OperatingLeaseLiability
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant