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Investments - (Available-for-sale investments with unrealized losses) (Details) - External Credit Rating, Investment Grade [Member] - USD ($) $ in Thousands | Dec. 31, 2024 | Mar. 31, 2024
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Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | $ 245,220 | $ 147,901
Fair Value, Twelve Months or Longer | 1,724,691 | 1,950,719
Fair Value, Total | 1,969,911 | 2,098,620
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (998) | (1,516)
Unrealized Loss, Twelve Months or Longer | (165,747) | (221,108)
Unrealized Loss, Total | (166,745) | (222,624)
US Treasury, Government and Corporate Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months |  | 1,888
Fair Value, Twelve Months or Longer | 99,405 | 103,336
Fair Value, Total | 99,405 | 105,224
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months |  | (13)
Unrealized Loss, Twelve Months or Longer | (6,094) | (8,908)
Unrealized Loss, Total | (6,094) | (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 | 37,449 |
Fair Value, Twelve Months or Longer | 22,153 | 23,711
Fair Value, Total | 59,602 | 23,711
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (210) |
Unrealized Loss, Twelve Months or Longer | (7,733) | (7,664)
Unrealized Loss, Total | (7,943) | (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 | 23,020 | 10,492
Fair Value, Twelve Months or Longer | 59,752 | 80,082
Fair Value, Total | 82,772 | 90,574
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (252) | (222)
Unrealized Loss, Twelve Months or Longer | (5,216) | (7,311)
Unrealized Loss, Total | (5,468) | (7,533)
Corporate Debt Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 120,515 | 132,513
Fair Value, Twelve Months or Longer | 1,314,484 | 1,495,167
Fair Value, Total | 1,434,999 | 1,627,680
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (330) | (1,258)
Unrealized Loss, Twelve Months or Longer | (111,361) | (151,541)
Unrealized Loss, Total | (111,691) | (152,799)
Collateralized Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 64,236 | 3,008
Fair Value, Twelve Months or Longer | 228,897 | 248,423
Fair Value, Total | 293,133 | 251,431
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (206) | (23)
Unrealized Loss, Twelve Months or Longer | (35,343) | (45,684)
Unrealized Loss, Total | $ (35,549) | $ (45,707)

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