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Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Mar. 31, 2018
Available-for-sale securities, amortized cost:    
Amortized cost $ 1,992,534 $ 1,823,199
Available-for-sale securities, fair value:    
Available for sale investments, estimated market value 1,980,720 1,883,384
US Treasury Securities [Member]    
Available-for-sale securities, amortized cost:    
Due in one year or less 54,102 36,446
Due after one year through five years 489,499 441,223
Due after five years through ten years 625,687 607,895
Due after ten years 720,106 641,411
Amortized cost by contractural maturity 1,889,394 1,726,975
Available-for-sale securities, fair value:    
Due in one year or less 54,349 36,674
Due after one year through five years 489,545 450,816
Due after five years through ten years 619,371 626,174
Due after ten years 714,570 671,448
Estimated market value by contractural maturity 1,877,835 1,785,112
Mortgage-backed securities [Member]    
Available-for-sale securities, amortized cost:    
Amortized cost 101,647 94,106
Available-for-sale securities, fair value:    
Available for sale investments, estimated market value 101,329 96,025
Redeemable preferred stock    
Available-for-sale securities, amortized cost:    
Amortized cost 1,493 2,118
Available-for-sale securities, fair value:    
Available for sale investments, estimated market value $ 1,556 $ 2,247