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Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($)
$ in Thousands
Sep. 30, 2020
Mar. 31, 2020
Available-for-sale securities, amortized cost:    
Amortized cost $ 2,263,299 $ 2,334,215
Available-for-sale securities, fair value:    
Estimated market value 2,459,159 2,467,613
US Treasury Securities [Member]    
Available-for-sale securities, amortized cost:    
Due in one year or less 112,348 128,747
Due after one year through five years 532,931 547,821
Due after five years through ten years 626,777 636,036
Due after ten years 790,921 832,334
Amortized cost 2,062,977 2,144,938
Available-for-sale securities, fair value:    
Due in one year or less 110,836 129,420
Due after one year through five years 562,558 566,934
Due after five years through ten years 691,999 678,636
Due after ten years 882,203 897,371
Estimated market value 2,247,596 2,272,361
Mortgage-backed securities [Member]    
Available-for-sale securities, amortized cost:    
Amortized cost 198,829 187,784
Available-for-sale securities, fair value:    
Estimated market value 210,033 193,687
Redeemable preferred stocks [Member]    
Available-for-sale securities, amortized cost:    
Amortized cost 1,493 1,493
Available-for-sale securities, fair value:    
Estimated market value $ 1,530 $ 1,565