Income Taxes (Details Textual) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Income Tax Disclosure [Abstract]
Unrecognized Tax Benefits	$ 5.3
Decrease in income tax expenses unrecognized tax benefits if recognized	5.3
Decrease in Income Tax Expenses Unrecognized Tax Benefits if Recognized Net of Federal Tax	3.4
Reductions due to expiration of the applicable statute of limitations	0.4
Effective Income Tax Rate Reconciliation, Percent	33.00%	45.00%
gmt_TaxOnGainOfAffiliateSale		23.2
gmt_AdjustedTaxRate		27.00%
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	35.00%	35.00%
Tax Credit Carryforward, Amount		$ (3.9)