v3.26.1
Investments (Available-For-Sale Investments) (Details) - USD ($)
$ in Thousands
Mar. 31, 2026
Dec. 31, 2025
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost $ 18,207,316  
Available-for-sale fixed maturity securities, estimated fair value 17,943,781 $ 17,797,986
Investments, available-for-sale, amortized cost 20,234,398 19,925,568
Investments, available-for-sale, gross unrealized holding gains 139,525 237,516
Investments, available-for-sale, gross unrealized holding losses (403,995) (331,436)
Investments, available-for-sale, estimated fair value 19,969,928 19,831,648
U.S. Treasury securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 5,992,327 5,877,779
Available-for-sale fixed maturity securities, gross unrealized holding gains 33,328 63,769
Available-for-sale fixed maturity securities, gross unrealized holding losses (23,853) (13,641)
Available-for-sale fixed maturity securities, estimated fair value 6,001,802 5,927,907
U.S. government-sponsored enterprises [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 1,715,840 1,678,599
Available-for-sale fixed maturity securities, gross unrealized holding gains 13,140 19,378
Available-for-sale fixed maturity securities, gross unrealized holding losses (64,358) (57,743)
Available-for-sale fixed maturity securities, estimated fair value 1,664,622 1,640,234
Obligations of states, municipalities and political subdivisions [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 3,759,474 3,743,154
Available-for-sale fixed maturity securities, gross unrealized holding gains 24,502 34,871
Available-for-sale fixed maturity securities, gross unrealized holding losses (111,523) (105,557)
Available-for-sale fixed maturity securities, estimated fair value 3,672,453 3,672,468
Foreign governments, agencies and supranationals [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 3,484,109 3,359,014
Available-for-sale fixed maturity securities, gross unrealized holding gains 51,406 89,595
Available-for-sale fixed maturity securities, gross unrealized holding losses (111,258) (73,694)
Available-for-sale fixed maturity securities, estimated fair value 3,424,257 3,374,915
Agency mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 2,978,312 2,935,758
Available-for-sale fixed maturity securities, gross unrealized holding gains 15,434 24,903
Available-for-sale fixed maturity securities, gross unrealized holding losses (66,982) (59,541)
Available-for-sale fixed maturity securities, estimated fair value 2,926,764 2,901,120
Non-agency mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 54,756 66,296
Available-for-sale fixed maturity securities, gross unrealized holding gains 0 0
Available-for-sale fixed maturity securities, gross unrealized holding losses (701) (538)
Available-for-sale fixed maturity securities, estimated fair value 54,055 65,758
Corporate and university bonds [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 222,498 235,318
Available-for-sale fixed maturity securities, gross unrealized holding gains 189 368
Available-for-sale fixed maturity securities, gross unrealized holding losses (22,859) (20,102)
Available-for-sale fixed maturity securities, estimated fair value 199,828 215,584
Total fixed maturity securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 18,207,316 17,895,918
Available-for-sale fixed maturity securities, gross unrealized holding gains 137,999 232,884
Available-for-sale fixed maturity securities, gross unrealized holding losses (401,534) (330,816)
Available-for-sale fixed maturity securities, estimated fair value 17,943,781 17,797,986
Short-term investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Short-term investments, available-for-sale, amortized cost 2,027,082 2,029,650
Short-term investments, available-for-sale, gross unrealized holding gains 1,526 4,632
Short-term investments, available-for-sale, gross unrealized holding losses (2,461) (620)
Short-term investments, available-for-sale, estimated fair value $ 2,026,147 $ 2,033,662