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Investments AFS, Debt Maturities (Details) - USD ($)
$ in Thousands
Sep. 30, 2015
Mar. 31, 2015
Schedule of available-for-sale securities, debt maturities [Line Items]    
Available-for-sale, adjusted cost $ 2,046,735 $ 1,719,843
Available-for-sale, gross unrealized gain 2,638 15,011
Available-for-sale, gross unrealized loss (1,997) (474)
Available-for-sale, estimated fair value 2,047,376 1,734,380
Due in one year or less [Member]    
Schedule of available-for-sale securities, debt maturities [Line Items]    
Available-for-sale, adjusted cost 377,682  
Available-for-sale, gross unrealized gain 650  
Available-for-sale, gross unrealized loss   (42)
Available-for-sale, estimated fair value 378,290  
Due after one year and through five years [Member]    
Schedule of available-for-sale securities, debt maturities [Line Items]    
Available-for-sale, adjusted cost 1,520,303  
Available-for-sale, gross unrealized gain 1,942  
Available-for-sale, gross unrealized loss   (1,954)
Available-for-sale, estimated fair value 1,520,291  
Due after five years and through ten years [Member]    
Schedule of available-for-sale securities, debt maturities [Line Items]    
Available-for-sale, adjusted cost 132,735  
Available-for-sale, gross unrealized gain 46  
Available-for-sale, gross unrealized loss   (1)
Available-for-sale, estimated fair value 132,780  
Due after ten years [Member]    
Schedule of available-for-sale securities, debt maturities [Line Items]    
Available-for-sale, adjusted cost 9,825  
Available-for-sale, gross unrealized gain 0  
Available-for-sale, gross unrealized loss   0
Available-for-sale, estimated fair value 9,825  
Total Maturities [Member]    
Schedule of available-for-sale securities, debt maturities [Line Items]    
Available-for-sale, adjusted cost 2,040,545  
Available-for-sale, gross unrealized gain 2,638  
Available-for-sale, gross unrealized loss   $ (1,997)
Available-for-sale, estimated fair value $ 2,041,186