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Investments - (Available-for-sale investments with unrealized losses) (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Mar. 31, 2024
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Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | $ 344,537 |
Fair Value, Twelve Months or Longer | 1,907,363 |
Fair Value, Total | 2,251,900 |
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (2,841) |
Unrealized Loss, Twelve Months or Longer | (241,185) |
Unrealized Loss, Total | (244,026) |
External Credit Rating, Investment Grade [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months |  | $ 147,901
Fair Value, Twelve Months or Longer |  | 1,950,719
Fair Value, Total |  | 2,098,620
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months |  | (1,516)
Unrealized Loss, Twelve Months or Longer |  | (221,108)
Unrealized Loss, Total |  | (222,624)
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 | 39,648 | 1,888
Fair Value, Twelve Months or Longer | 97,088 | 103,336
Fair Value, Total | 136,736 | 105,224
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (35) | (13)
Unrealized Loss, Twelve Months or Longer | (9,313) | (8,908)
Unrealized Loss, Total | (9,348) | (8,921)
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, Twelve Months or Longer | 22,087 | 23,711
Fair Value, Total | 22,087 | 23,711
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Twelve Months or Longer | (8,255) | (7,664)
Unrealized Loss, Total | (8,255) | (7,664)
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 | 37,607 | 10,492
Fair Value, Twelve Months or Longer | 79,068 | 80,082
Fair Value, Total | 116,675 | 90,574
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (676) | (222)
Unrealized Loss, Twelve Months or Longer | (8,422) | (7,311)
Unrealized Loss, Total | (9,098) | (7,533)
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 | 208,344 | 132,513
Fair Value, Twelve Months or Longer | 1,475,083 | 1,495,167
Fair Value, Total | 1,683,427 | 1,627,680
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (1,953) | (1,258)
Unrealized Loss, Twelve Months or Longer | (169,655) | (151,541)
Unrealized Loss, Total | (171,608) | (152,799)
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 | 58,938 | 3,008
Fair Value, Twelve Months or Longer | 234,037 | 248,423
Fair Value, Total | 292,975 | 251,431
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (177) | (23)
Unrealized Loss, Twelve Months or Longer | (45,540) | (45,684)
Unrealized Loss, Total | $ (45,717) | $ (45,707)

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