Assets and Liabilities, at Fair Value - Available for Sale Securities in OCI (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Assets and Liabilities, at Fair Value [Abstract]
Equity Investments in Exchange Stock - Cost	$ 3.7		$ 4.4
Equity Investments in Exchange Stock - Fair Value	4.4		12.4
Proceeds from Sale of Available-for-sale Securities	8.6
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax - LME Shares		6.3
Deferred Tax Liabilities, Accumulated Other Comprehensive Income - LME Shares		2.0
Proceeds from Shares of Exchanges Sold	1.5
Gain (Loss) on Sale of Exchange Shares	0.9
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	0.5		(6.2)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	$ (0.3)		$ 2.0