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Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Mar. 31, 2019
Available-for-sale securities, amortized cost:    
Amortized cost $ 2,299,673 $ 2,228,961
Available-for-sale securities, fair value:    
Estimated market value 2,441,979 2,211,229
US Treasury Securities [Member]    
Available-for-sale securities, amortized cost:    
Due in one year or less 114,557 71,987
Due after one year through five years 554,591 541,195
Due after five years through ten years 662,534 621,031
Due after ten years 812,618 845,052
Amortized cost 2,144,300 2,079,265
Available-for-sale securities, fair value:    
Due in one year or less 115,001 71,954
Due after one year through five years 572,647 540,658
Due after five years through ten years 707,015 614,485
Due after ten years 884,262 834,769
Estimated market value 2,278,925 2,061,866
Mortgage-backed securities [Member]    
Available-for-sale securities, amortized cost:    
Amortized cost 153,880 148,203
Available-for-sale securities, fair value:    
Estimated market value 161,479 147,895
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 $ 1,575 $ 1,468