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AFS, Debt Maturities (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Mar. 31, 2017
Schedule of available-for-sale securities, debt maturities [Line Items]    
Available-for-sale, adjusted cost $ 1,293.2 $ 501.0
Available-for-sale, gross unrealized gains 2.1 1.0
Available-for-sale, gross unrealized losses 0.0 (0.4)
Available-for-sale, estimated fair value 1,295.3 501.6
Due in one year or less    
Schedule of available-for-sale securities, debt maturities [Line Items]    
Available-for-sale, adjusted cost 246.5 342.7
Available-for-sale, gross unrealized gains 0.0 0.0
Available-for-sale, gross unrealized losses 0.0 (0.2)
Available-for-sale, estimated fair value 246.5 342.5
Due after one year and through five years    
Schedule of available-for-sale securities, debt maturities [Line Items]    
Available-for-sale, adjusted cost 1,046.0 157.6
Available-for-sale, gross unrealized gains 0.0 0.1
Available-for-sale, gross unrealized losses 0.0 (0.2)
Available-for-sale, estimated fair value 1,046.0 157.5
Due after five years and through ten years    
Schedule of available-for-sale securities, debt maturities [Line Items]    
Available-for-sale, adjusted cost 0.0 0.0
Available-for-sale, gross unrealized gains 0.0 0.0
Available-for-sale, gross unrealized losses 0.0 0.0
Available-for-sale, estimated fair value 0.0 0.0
Due after ten years    
Schedule of available-for-sale securities, debt maturities [Line Items]    
Available-for-sale, adjusted cost 0.0 0.0
Available-for-sale, gross unrealized gains 0.0 0.0
Available-for-sale, gross unrealized losses 0.0 0.0
Available-for-sale, estimated fair value 0.0 0.0
Total Maturities    
Schedule of available-for-sale securities, debt maturities [Line Items]    
Available-for-sale, adjusted cost 1,292.5 500.3
Available-for-sale, gross unrealized gains 0.0 0.1
Available-for-sale, gross unrealized losses 0.0 (0.4)
Available-for-sale, estimated fair value $ 1,292.5 $ 500.0