v3.25.2
Investments (Available-For-Sale Investments) (Details) - USD ($)
$ in Thousands
Jun. 30, 2025
Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost $ 16,959,574  
Available-for-sale fixed maturity securities, estimated fair value 16,772,408 $ 15,745,539
Investments, available-for-sale, amortized cost 18,849,591 18,988,664
Investments, available-for-sale, gross unrealized holding gains 223,644 24,788
Investments, available-for-sale, gross unrealized holding losses (406,303) (743,003)
Investments, available-for-sale, estimated fair value 18,666,932 18,270,449
U.S. Treasury securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 5,474,824 5,147,365
Available-for-sale fixed maturity securities, gross unrealized holding gains 53,829 8,962
Available-for-sale fixed maturity securities, gross unrealized holding losses (24,453) (68,469)
Available-for-sale fixed maturity securities, estimated fair value 5,504,200 5,087,858
U.S. government-sponsored enterprises [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 1,576,299 1,445,171
Available-for-sale fixed maturity securities, gross unrealized holding gains 15,402 2,976
Available-for-sale fixed maturity securities, gross unrealized holding losses (68,703) (101,911)
Available-for-sale fixed maturity securities, estimated fair value 1,522,998 1,346,236
Obligations of states, municipalities and political subdivisions [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 3,764,932 3,813,146
Available-for-sale fixed maturity securities, gross unrealized holding gains 21,514 5,866
Available-for-sale fixed maturity securities, gross unrealized holding losses (136,284) (199,520)
Available-for-sale fixed maturity securities, estimated fair value 3,650,162 3,619,492
Foreign governments, agencies and supranationals [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 3,090,058 2,909,561
Available-for-sale fixed maturity securities, gross unrealized holding gains 109,023 4,264
Available-for-sale fixed maturity securities, gross unrealized holding losses (73,786) (207,302)
Available-for-sale fixed maturity securities, estimated fair value 3,125,295 2,706,523
Agency mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 2,747,479 2,771,589
Available-for-sale fixed maturity securities, gross unrealized holding gains 18,940 2,096
Available-for-sale fixed maturity securities, gross unrealized holding losses (79,158) (123,872)
Available-for-sale fixed maturity securities, estimated fair value 2,687,261 2,649,813
Non-agency mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 71,621 122,373
Available-for-sale fixed maturity securities, gross unrealized holding gains 0 0
Available-for-sale fixed maturity securities, gross unrealized holding losses (1,264) (4,343)
Available-for-sale fixed maturity securities, estimated fair value 70,357 118,030
Corporate and university bonds [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 234,361 248,518
Available-for-sale fixed maturity securities, gross unrealized holding gains 176 76
Available-for-sale fixed maturity securities, gross unrealized holding losses (22,402) (31,007)
Available-for-sale fixed maturity securities, estimated fair value 212,135 217,587
Total fixed maturity securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 16,959,574 16,457,723
Available-for-sale fixed maturity securities, gross unrealized holding gains 218,884 24,240
Available-for-sale fixed maturity securities, gross unrealized holding losses (406,050) (736,424)
Available-for-sale fixed maturity securities, estimated fair value 16,772,408 15,745,539
Short-term investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Short-term investments, available-for-sale, amortized cost 1,890,017 2,530,941
Short-term investments, available-for-sale, gross unrealized holding gains 4,760 548
Short-term investments, available-for-sale, gross unrealized holding losses (253) (6,579)
Short-term investments, available-for-sale, estimated fair value $ 1,894,524 $ 2,524,910