Debt Operating assets pledged as collateral (Details) $ in Millions	Dec. 31, 2016 USD ($)
Debt Disclosure [Abstract]
Operating Assets Pledged as Collateral for Notes or Other Obligations	$ 301.1

Operating assets pledged as collateral for notes or other obligations.

No definition available.

Name:	gmt_OperatingAssetsPledgedAsCollateralForNotesOrOtherObligations
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant