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Investments (Available For Sale Investments) (Details) - USD ($)
$ in Thousands
Jun. 30, 2022
Mar. 31, 2022
Available-for-sale Securities [Abstract]    
Amortized cost $ 2,640,198 $ 2,664,037
Gross Unrealized Gains 49,434 170,294
Gross unrealized losses more than 12 months (14,493) (445)
Gross unrealized losses less than 12 months (76,863) (12,734)
Allowance for Expected Credit Losses (77) (60)
Estimated market value 2,598,199 2,821,092
U.S. treasury securities and government obligations [Member]    
Available-for-sale Securities [Abstract]    
Amortized cost 127,940 128,078
Gross Unrealized Gains 2,947 7,984
Gross unrealized losses more than 12 months (1,578) 0
Gross unrealized losses less than 12 months (2,493) (969)
Allowance for Expected Credit Losses 0 0
Estimated market value 126,816 135,093
U.S. government agency mortgage-backed securities [Member]    
Available-for-sale Securities [Abstract]    
Amortized cost 37,806 44,678
Gross Unrealized Gains 149 280
Gross unrealized losses more than 12 months (380) (42)
Gross unrealized losses less than 12 months (4,126) (3,111)
Allowance for Expected Credit Losses 0 0
Estimated market value 33,449 41,805
Obligations of states and political subdivisions [Member]    
Available-for-sale Securities [Abstract]    
Amortized cost 166,572 178,040
Gross Unrealized Gains 5,830 15,450
Gross unrealized losses more than 12 months (1,275) 0
Gross unrealized losses less than 12 months (2,009) (508)
Allowance for Expected Credit Losses 0 0
Estimated market value 169,118 192,982
Corporate securities [Member]    
Available-for-sale Securities [Abstract]    
Amortized cost 1,970,857 1,989,212
Gross Unrealized Gains 38,302 138,909
Gross unrealized losses more than 12 months (11,259) (402)
Gross unrealized losses less than 12 months (51,863) (6,604)
Allowance for Expected Credit Losses (77) (60)
Estimated market value 1,945,960 2,121,055
Mortgage-backed securities [Member]    
Available-for-sale Securities [Abstract]    
Amortized cost 337,023 324,029
Gross Unrealized Gains 2,206 7,671
Gross unrealized losses more than 12 months (1) (1)
Gross unrealized losses less than 12 months (16,372) (1,542)
Allowance for Expected Credit Losses 0 0
Estimated market value $ 322,856 $ 330,157