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Investments in Marketable Securities (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Cost of investments in marketable securities, gross unrealized gains and losses    
Total investment $ 199.9 $ 172.6
Realized gains (losses) on available-for-sale securities $ 8.8 $ 3.4