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Investments in Marketable Securities - Additional Information (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Cost of investments in marketable securities, gross unrealized gains and losses    
Proceeds from sale of investment classified as available-for-sale $ 82.3 $ 61.1
Realized net gains on available-for-sale securities 29.2 19.2
Proceeds from trading securities 29.9 82.8
Realized net gains on trading securities $ 6.6 $ 1.0