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Investments - (Available-for-sale investments with unrealized losses) (Details) - External Credit Rating, Investment Grade [Member] - USD ($)
$ in Thousands
Mar. 31, 2025
Mar. 31, 2024
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]    
Fair Value, Less than Twelve Months $ 567,128 $ 147,901
Fair Value, Twelve Months or Longer 1,625,793 1,950,719
Fair Value, Total 2,192,921 2,098,620
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]    
Unrealized Loss, Less than Twelve Months (11,582) (1,516)
Unrealized Loss, Twelve Months or Longer (217,293) (221,108)
Unrealized Loss, Total (228,875) (222,624)
US Treasury, Government and Corporate Securities [Member]    
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]    
Fair Value, Less than Twelve Months 1,760 1,888
Fair Value, Twelve Months or Longer 95,058 103,336
Fair Value, Total 96,818 105,224
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]    
Unrealized Loss, Less than Twelve Months (24) (13)
Unrealized Loss, Twelve Months or Longer (8,329) (8,908)
Unrealized Loss, Total (8,353) (8,921)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]    
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]    
Fair Value, Less than Twelve Months 36,871  
Fair Value, Twelve Months or Longer 20,928 23,711
Fair Value, Total 57,799 23,711
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]    
Unrealized Loss, Less than Twelve Months (197)  
Unrealized Loss, Twelve Months or Longer (8,515) (7,664)
Unrealized Loss, Total (8,712) (7,664)
US States and Political Subdivisions Debt Securities [Member]    
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]    
Fair Value, Less than Twelve Months 46,036 10,492
Fair Value, Twelve Months or Longer 52,903 80,082
Fair Value, Total 98,939 90,574
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]    
Unrealized Loss, Less than Twelve Months (1,628) (222)
Unrealized Loss, Twelve Months or Longer (7,179) (7,311)
Unrealized Loss, Total (8,807) (7,533)
Corporate Debt Securities [Member]    
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]    
Fair Value, Less than Twelve Months 294,133 132,513
Fair Value, Twelve Months or Longer 1,239,884 1,495,167
Fair Value, Total 1,534,017 1,627,680
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]    
Unrealized Loss, Less than Twelve Months (5,822) (1,258)
Unrealized Loss, Twelve Months or Longer (149,927) (151,541)
Unrealized Loss, Total (155,749) (152,799)
Collateralized Mortgage-Backed Securities [Member]    
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]    
Fair Value, Less than Twelve Months 188,328 3,008
Fair Value, Twelve Months or Longer 217,020 248,423
Fair Value, Total 405,348 251,431
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]    
Unrealized Loss, Less than Twelve Months (3,911) (23)
Unrealized Loss, Twelve Months or Longer (43,343) (45,684)
Unrealized Loss, Total $ (47,254) $ (45,707)