v3.25.4
Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($)
$ in Thousands
Dec. 31, 2025
Mar. 31, 2025
Available-for-sale securities, amortized cost:    
Amortized cost $ 2,649,214 $ 2,708,562
Available-for-sale securities, fair value:    
Available for sale investments, estimated market value 2,501,436 2,479,498
US Treasury Securities [Member]    
Available-for-sale securities, amortized cost:    
Due in one year or less 176,399 196,238
Due after one year through five years 577,465 591,589
Due after five years through ten years 547,894 611,788
Due after ten years 753,501 746,468
Amortized cost 2,055,259 2,146,083
Available-for-sale securities, fair value:    
Due in one year or less 175,753 194,896
Due after one year through five years 568,566 576,204
Due after five years through ten years 518,739 558,430
Due after ten years 674,410 634,160
Available for sale investments, estimated market value 1,937,468 1,963,690
Mortgage-backed securities [Member]    
Available-for-sale securities, amortized cost:    
Amortized cost 593,955 562,479
Available-for-sale securities, fair value:    
Available for sale investments, estimated market value $ 563,968 $ 515,808