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Investments (Investments by Category) (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Mar. 31, 2017
Equity Method Investments, Cost Method Investments, and Investments in Debt and Equity [Abstract]    
Equity method investments $ 137.0 $ 322.9
Available-for-sale securities 12.8 8.0
Cost method investments 40.6 40.6
Investments $ 190.4 $ 371.5