Income Taxes (Details Textual) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating Loss Carryforwards [Line Items]
Effective Income Tax Rate Reconciliation, Percent	41.20%	18.20%	27.10%
gmt_TaxOnGainOfAffiliateSale	$ (23.2)	$ 0	$ 0
Cumulative amount of undistributed earning of foreign subsidiaries	621.4
Alternative minimum tax credit	8.4	5.2
Foreign tax credit	3.6	7.3
Foreign tax credits valuation allowance	3.6	7.3
State net operating loss	27.1	26.6
State net operating loss carryforwards valuation allowance	11.2	12.8
Foreign net operating loss	8.5	9.5
Unrecognized Tax Benefits	5.7	4.7	20.8
Decrease in income tax expenses unrecognized tax benefits if recognized	3.9	3.2
Unrecognized foreign tax benefits	0.4
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	1.0	0
Reductions due to expiration of the applicable statute of limitations	0	16.1
Expiration of the applicable statute of limitations	(0.6)	15.5	0
Domestic [Member]
Operating Loss Carryforwards [Line Items]
Amount of U.S. federal tax net operating loss	25.4
Foreign [Member]
Operating Loss Carryforwards [Line Items]
Amount of U.S. federal tax net operating loss	$ 8.3