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Investments in Marketable Securities (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Cost of investments in marketable securities, gross unrealized gains and losses    
Total investment $ 172.6us-gaap_MarketableSecurities $ 157.9us-gaap_MarketableSecurities
Realized gains (losses) on available-for-sale securities $ 3.4us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet $ 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet