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Investments (Investments by Category) (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Mar. 31, 2018
Equity Method Investments, Cost Method Investments, and Investments in Debt and Equity [Abstract]    
Equity method investments $ 124.7 $ 127.0
Available-for-sale securities 4.6 7.3
Cost method investments 0.5 30.6
Investments $ 129.8 $ 164.9