v3.25.1
Investments (Available-For-Sale Investments) (Details) - USD ($)
$ in Thousands
Mar. 31, 2025
Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost $ 16,716,601  
Available-for-sale fixed maturity securities, estimated fair value 16,277,629 $ 15,745,539
Investments, available-for-sale, amortized cost 18,817,651 18,988,664
Investments, available-for-sale, gross unrealized holding gains 105,889 24,788
Investments, available-for-sale, gross unrealized holding losses (546,878) (743,003)
Investments, available-for-sale, estimated fair value 18,376,662 18,270,449
U.S. Treasury securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 5,257,869 5,147,365
Available-for-sale fixed maturity securities, gross unrealized holding gains 39,374 8,962
Available-for-sale fixed maturity securities, gross unrealized holding losses (33,562) (68,469)
Available-for-sale fixed maturity securities, estimated fair value 5,263,681 5,087,858
U.S. government-sponsored enterprises [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 1,495,738 1,445,171
Available-for-sale fixed maturity securities, gross unrealized holding gains 11,133 2,976
Available-for-sale fixed maturity securities, gross unrealized holding losses (77,553) (101,911)
Available-for-sale fixed maturity securities, estimated fair value 1,429,318 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 3,777,894 3,813,146
Available-for-sale fixed maturity securities, gross unrealized holding gains 15,465 5,866
Available-for-sale fixed maturity securities, gross unrealized holding losses (153,987) (199,520)
Available-for-sale fixed maturity securities, estimated fair value 3,639,372 3,619,492
Foreign governments, agencies and supranationals [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 3,067,330 2,909,561
Available-for-sale fixed maturity securities, gross unrealized holding gains 26,273 4,264
Available-for-sale fixed maturity securities, gross unrealized holding losses (160,800) (207,302)
Available-for-sale fixed maturity securities, estimated fair value 2,932,803 2,706,523
Agency mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 2,776,864 2,771,589
Available-for-sale fixed maturity securities, gross unrealized holding gains 13,005 2,096
Available-for-sale fixed maturity securities, gross unrealized holding losses (90,997) (123,872)
Available-for-sale fixed maturity securities, estimated fair value 2,698,872 2,649,813
Non-agency mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 104,737 122,373
Available-for-sale fixed maturity securities, gross unrealized holding gains 0 0
Available-for-sale fixed maturity securities, gross unrealized holding losses (2,783) (4,343)
Available-for-sale fixed maturity securities, estimated fair value 101,954 118,030
Corporate and university bonds [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 236,169 248,518
Available-for-sale fixed maturity securities, gross unrealized holding gains 268 76
Available-for-sale fixed maturity securities, gross unrealized holding losses (24,808) (31,007)
Available-for-sale fixed maturity securities, estimated fair value 211,629 217,587
Total fixed maturity securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, available-for-sale, amortized cost 16,716,601 16,457,723
Available-for-sale fixed maturity securities, gross unrealized holding gains 105,518 24,240
Available-for-sale fixed maturity securities, gross unrealized holding losses (544,490) (736,424)
Available-for-sale fixed maturity securities, estimated fair value 16,277,629 15,745,539
Short-term investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Short-term investments, available-for-sale, amortized cost 2,101,050 2,530,941
Short-term investments, available-for-sale, gross unrealized holding gains 371 548
Short-term investments, available-for-sale, gross unrealized holding losses (2,388) (6,579)
Short-term investments, available-for-sale, estimated fair value $ 2,099,033 $ 2,524,910