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Investments in Marketable Securities - Additional Information (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Cost of investments in marketable securities, gross unrealized gains and losses    
Investments in marketable securities $ 122.4 $ 199.9
Proceeds from sale of investment classified as available-for-sale 61.1 18.1
Realized gains (losses) on available-for-sale securities 19.9 8.8
Realized gains on trading securities 4.4 0.8
Realized losses on trading securities $ 3.4 $ 0.1