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Investments in Marketable Securities (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Marketable Securities [Abstract]          
Investments in marketable securities $ 171.8   $ 171.8   $ 172.6
Realized gains on investments classified as available-for-sale $ 0.7 $ 1.8 $ 8.7 $ 1.8