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Investments (Available-for-sale Securities) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]        
Unrealized losses on available-for-sale-securities held $ 1.9 $ 0.0 $ 2.8 $ 0.0