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Investments - (Available-for-sale investments with unrealized losses) (Details) - USD ($) $ in Thousands | Jun. 30, 2025 | Mar. 31, 2025
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Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | $ 432,615 |
Fair Value, Twelve Months or Longer | 1,591,378 |
Fair Value, Total | 2,023,993 |
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (5,534) |
Unrealized Loss, Twelve Months or Longer | (189,169) |
Unrealized Loss, Total | (194,703) |
External Credit Rating, Investment Grade [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months |  | $ 567,128
Fair Value, Twelve Months or Longer |  | 1,625,793
Fair Value, Total |  | 2,192,921
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months |  | (11,582)
Unrealized Loss, Twelve Months or Longer |  | (217,293)
Unrealized Loss, Total |  | (228,875)
US Treasury, Government and Corporate Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Total | 97,116 |
US Treasury, Government and Corporate Securities [Member] | External Credit Rating, Investment Grade [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 1,459 | 1,760
Fair Value, Twelve Months or Longer | 95,657 | 95,058
Fair Value, Total |  | 96,818
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (2) | (24)
Unrealized Loss, Twelve Months or Longer | (6,627) | (8,329)
Unrealized Loss, Total | (6,629) | (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 | 22,220 |
Fair Value, Total | 42,968 |
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (143) |
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | External Credit Rating, Investment Grade [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months |  | 36,871
Fair Value, Twelve Months or Longer | 20,748 | 20,928
Fair Value, Total |  | 57,799
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months |  | (197)
Unrealized Loss, Twelve Months or Longer | (8,205) | (8,515)
Unrealized Loss, Total | (8,348) | (8,712)
US States and Political Subdivisions Debt Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Total | 95,439 |
US States and Political Subdivisions Debt Securities [Member] | External Credit Rating, Investment Grade [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 41,243 | 46,036
Fair Value, Twelve Months or Longer | 54,196 | 52,903
Fair Value, Total |  | 98,939
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (1,530) | (1,628)
Unrealized Loss, Twelve Months or Longer | (6,729) | (7,179)
Unrealized Loss, Total | (8,259) | (8,807)
Corporate Debt Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Total | 1,439,687 |
Corporate Debt Securities [Member] | External Credit Rating, Investment Grade [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 229,829 | 294,133
Fair Value, Twelve Months or Longer | 1,209,858 | 1,239,884
Fair Value, Total |  | 1,534,017
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (2,724) | (5,822)
Unrealized Loss, Twelve Months or Longer | (130,475) | (149,927)
Unrealized Loss, Total | (133,199) | (155,749)
Collateralized Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Total | 348,783 |
Collateralized Mortgage Backed Securities [Member] | External Credit Rating, Investment Grade [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 137,864 | 188,328
Fair Value, Twelve Months or Longer | 210,919 | 217,020
Fair Value, Total |  | 405,348
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (1,135) | (3,911)
Unrealized Loss, Twelve Months or Longer | (37,133) | (43,343)
Unrealized Loss, Total | $ (38,268) | $ (47,254)

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