Loans Loans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2013	Dec. 31, 2012
Loans Receivable, Net [Abstract]
2014	$ 98.3
2015	6.3
2016	6.1
2017	3.2
2018	8.8
Thereafter	0
Loans receivables, outstanding	$ 122.7	$ 27.2

Loans receivables gross carrying amount.

No definition available.

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

Future loan payments receivable within one year of the balance sheet date.

No definition available.

Name:	gmt_Usgaap_LoanReceivablesFutureMinimumPaymentsCurrent
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

Future loan payments receivable within five years of the balance sheet date.

No definition available.

Name:	gmt_Usgaap_LoanReceivablesFutureMinimumPaymentsInFiveYears
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

Future loan payments receivable within four years of the balance sheet date.

No definition available.

Name:	gmt_Usgaap_LoanReceivablesFutureMinimumPaymentsInFourYears
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

Future loan payments receivable within three years of the balance sheet date.

No definition available.

Name:	gmt_Usgaap_LoanReceivablesFutureMinimumPaymentsInThreeYears
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

Future loan payments receivable within two years of the balance sheet date.

No definition available.

Name:	gmt_Usgaap_LoanReceivablesFutureMinimumPaymentsInTwoYears
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

Future loan payments receivable greater than five years of the balance sheet date.

No definition available.

Name:	gmt_Usgaap_LoanReceivablesFutureMinimumPaymentsThereafter
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_LoansReceivableNetAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration