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Investments (Summary Of Gross Unrealized Investment Losses By Length Of Time That Securities Have Continuously Been In An Unrealized Loss Position) (Details) - USD ($)
$ in Thousands
Sep. 30, 2022
Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months $ 9,504,639 $ 4,115,294
Available-for-sale securities, gross unrealized holding losses, less than 12 months (788,925) (57,145)
Available-for-sale securities, estimated fair value, 12 months or longer 2,758,101 169,794
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (350,978) (6,048)
Available-for-sale securities, estimated fair value 12,262,740 4,285,088
Available-for-sale securities, gross unrealized holding losses (1,139,903) (63,193)
Short-term investments, estimated fair value, less than 12 months 1,107,053 228,870
Short-term investments, gross unrealized holding losses, less than 12 months (17,022) (5,340)
Short-term investments, estimated fair value, 12 months or longer 0 0
Short-term investments, gross unrealized holding losses, 12 months or longer 0 0
Short-term investments, estimated fair value 1,107,053 228,870
Short-term investments, gross unrealized holding losses (17,022) (5,340)
U.S. Treasury securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 1,208,818 2,236,637
Available-for-sale securities, gross unrealized holding losses, less than 12 months (59,585) (18,433)
Available-for-sale securities, estimated fair value, 12 months or longer 1,749,603 97,173
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (127,081) (3,038)
Available-for-sale securities, estimated fair value 2,958,421 2,333,810
Available-for-sale securities, gross unrealized holding losses (186,666) (21,471)
U.S. government-sponsored enterprises [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 486,025 381,495
Available-for-sale securities, gross unrealized holding losses, less than 12 months (63,103) (5,640)
Available-for-sale securities, estimated fair value, 12 months or longer 224,076 14,010
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (50,275) (799)
Available-for-sale securities, estimated fair value 710,101 395,505
Available-for-sale securities, gross unrealized holding losses (113,378) (6,439)
Obligations of states, municipalities and political subdivisions [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 2,955,371 393,249
Available-for-sale securities, gross unrealized holding losses, less than 12 months (205,139) (6,941)
Available-for-sale securities, estimated fair value, 12 months or longer 253,651 23,589
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (60,843) (921)
Available-for-sale securities, estimated fair value 3,209,022 416,838
Available-for-sale securities, gross unrealized holding losses (265,982) (7,862)
Foreign governments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 1,006,658 322,813
Available-for-sale securities, gross unrealized holding losses, less than 12 months (177,900) (8,596)
Available-for-sale securities, estimated fair value, 12 months or longer 166,232 25,564
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (37,432) (892)
Available-for-sale securities, estimated fair value 1,172,890 348,377
Available-for-sale securities, gross unrealized holding losses (215,332) (9,488)
Commercial mortgage-backed securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 1,677,058 345,616
Available-for-sale securities, gross unrealized holding losses, less than 12 months (122,811) (7,765)
Available-for-sale securities, estimated fair value, 12 months or longer 255,181 9,189
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (48,432) (387)
Available-for-sale securities, estimated fair value 1,932,239 354,805
Available-for-sale securities, gross unrealized holding losses (171,243) (8,152)
Residential mortgage-backed securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 510,227 12,828
Available-for-sale securities, gross unrealized holding losses, less than 12 months (24,860) (159)
Available-for-sale securities, estimated fair value, 12 months or longer 3,899 269
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (767) (11)
Available-for-sale securities, estimated fair value 514,126 13,097
Available-for-sale securities, gross unrealized holding losses (25,627) (170)
Asset-backed securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 2,012  
Available-for-sale securities, gross unrealized holding losses, less than 12 months (54)  
Available-for-sale securities, estimated fair value, 12 months or longer 0  
Available-for-sale securities, gross unrealized holding losses, 12 months or longer 0  
Available-for-sale securities, estimated fair value 2,012  
Available-for-sale securities, gross unrealized holding losses (54)  
Corporate bonds [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 551,417 193,786
Available-for-sale securities, gross unrealized holding losses, less than 12 months (118,451) (4,271)
Available-for-sale securities, estimated fair value, 12 months or longer 105,459 0
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (26,148) 0
Available-for-sale securities, estimated fair value 656,876 193,786
Available-for-sale securities, gross unrealized holding losses (144,599) (4,271)
Total fixed maturities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 8,397,586 3,886,424
Available-for-sale securities, gross unrealized holding losses, less than 12 months (771,903) (51,805)
Available-for-sale securities, estimated fair value, 12 months or longer 2,758,101 169,794
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (350,978) (6,048)
Available-for-sale securities, estimated fair value 11,155,687 4,056,218
Available-for-sale securities, gross unrealized holding losses $ (1,122,881) $ (57,853)