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Investments (Available For Sale Investments) (Details) - USD ($)
$ in Thousands
Dec. 31, 2023
Mar. 31, 2023
Available-for-sale Securities [Abstract]    
Debt Securities, Available-for-Sale, Amortized Cost, Current $ 2,816,481 $ 3,006,587
Available For Sale Debt Securities Accumulated Gross Unrealized Gain 3,258 5,319
Gross unrealized losses more than 12 months 343,411 300,768
Allowance for expected credit loss (1,704) (2,101)
Available for sale investments, estimated market value 2,474,624 2,709,037
U.S. treasury securities and government obligations [Member]    
Available-for-sale Securities [Abstract]    
Debt Securities, Available-for-Sale, Amortized Cost, Current 319,497 353,189
Available For Sale Debt Securities Accumulated Gross Unrealized Gain 2,471 3,061
Gross unrealized losses more than 12 months 13,023 11,574
Allowance for expected credit loss 0 0
Available for sale investments, estimated market value 308,945 344,676
U.S. government agency mortgage-backed securities [Member]    
Available-for-sale Securities [Abstract]    
Debt Securities, Available-for-Sale, Amortized Cost, Current 68,501 34,126
Available For Sale Debt Securities Accumulated Gross Unrealized Gain 149 40
Gross unrealized losses more than 12 months 8,353 6,935
Allowance for expected credit loss 0 0
Available for sale investments, estimated market value 60,297 27,231
Obligations of states and political subdivisions [Member]    
Available-for-sale Securities [Abstract]    
Debt Securities, Available-for-Sale, Amortized Cost, Current 154,660 161,960
Available For Sale Debt Securities Accumulated Gross Unrealized Gain 102 649
Gross unrealized losses more than 12 months 16,060 12,104
Allowance for expected credit loss 0 0
Available for sale investments, estimated market value 138,702 150,505
Corporate securities [Member]    
Available-for-sale Securities [Abstract]    
Debt Securities, Available-for-Sale, Amortized Cost, Current 1,943,321 2,086,432
Available For Sale Debt Securities Accumulated Gross Unrealized Gain 524 1,491
Gross unrealized losses more than 12 months 247,483 216,589
Allowance for expected credit loss (1,704) (2,101)
Available for sale investments, estimated market value 1,694,658 1,869,233
Mortgage-backed securities [Member]    
Available-for-sale Securities [Abstract]    
Debt Securities, Available-for-Sale, Amortized Cost, Current 330,502 370,880
Available For Sale Debt Securities Accumulated Gross Unrealized Gain 12 78
Gross unrealized losses more than 12 months 58,492 53,566
Allowance for expected credit loss 0 0
Available for sale investments, estimated market value $ 272,022 $ 317,392