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Investments - (Available-for-sale investments with unrealized losses) (Details) - External Credit Rating, Investment Grade [Member] - USD ($) $ in Thousands | Sep. 30, 2025 | Mar. 31, 2025
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Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | $ 327,188 | $ 567,128
Fair Value, Twelve Months or Longer | 1,547,413 | 1,625,793
Fair Value, Total | 1,874,601 | 2,192,921
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (5,488) | (11,582)
Unrealized Loss, Twelve Months or Longer | (174,225) | (217,293)
Unrealized Loss, Total | (179,713) | (228,875)
US Treasury, Government and Corporate Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months |  | 1,760
Fair Value, Twelve Months or Longer | 94,669 | 95,058
Fair Value, Total | 94,669 | 96,818
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months |  | (24)
Unrealized Loss, Twelve Months or Longer | (6,012) | (8,329)
Unrealized Loss, Total | (6,012) | (8,353)
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 | 10,695 | 36,871
Fair Value, Twelve Months or Longer | 19,921 | 20,928
Fair Value, Total | 30,616 | 57,799
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (46) | (197)
Unrealized Loss, Twelve Months or Longer | (7,486) | (8,515)
Unrealized Loss, Total | (7,532) | (8,712)
US States and Political Subdivisions Debt Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 41,058 | 46,036
Fair Value, Twelve Months or Longer | 53,467 | 52,903
Fair Value, Total | 94,525 | 98,939
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (1,439) | (1,628)
Unrealized Loss, Twelve Months or Longer | (6,858) | (7,179)
Unrealized Loss, Total | (8,297) | (8,807)
Corporate Debt Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 186,091 | 294,133
Fair Value, Twelve Months or Longer | 1,181,934 | 1,239,884
Fair Value, Total | 1,368,025 | 1,534,017
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (1,651) | (5,822)
Unrealized Loss, Twelve Months or Longer | (119,051) | (149,927)
Unrealized Loss, Total | (120,702) | (155,749)
Collateralized Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 89,344 | 188,328
Fair Value, Twelve Months or Longer | 197,422 | 217,020
Fair Value, Total | 286,766 | 405,348
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (2,352) | (3,911)
Unrealized Loss, Twelve Months or Longer | (34,818) | (43,343)
Unrealized Loss, Total | $ (37,170) | $ (47,254)

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