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Investments in Marketable Securities (Details 2) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Dec. 31, 2012
Available-for-Sale      
Cost $ 99.2   $ 103.2
Unrealized Gains 21.1   15.3
Unrealized Losses (3.6)   (3.2)
Fair Value 116.7   115.3
Trading      
Cost 16.3   10.3
Unrealized Gains 2.5   6.5
Unrealized Losses     (3.2)
Fair Value 18.8   13.6
Realized gains and losses on investments classified as available-for-sale      
Gains 0.2    
Net realized gains 0.2    
Realized gains and losses on investments classified as trading      
Gains 0.1 0.5  
Losses   (0.3)  
Net realized gains $ 0.1 $ 0.2