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Investments - (Available-for-sale investments with unrealized losses) (Details) - External Credit Rating, Investment Grade [Member] - USD ($) $ in Thousands | Jun. 30, 2024 | Mar. 31, 2024
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Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | $ 281,425 | $ 147,901
Fair Value, Twelve Months or Longer | 1,946,741 | 1,950,719
Fair Value, Total | 2,228,166 | 2,098,620
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (2,301) | (1,516)
Unrealized Loss, Twelve Months or Longer | (232,747) | (221,108)
Unrealized Loss, Total | (235,048) | (222,624)
US Treasury, Government and Corporate Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 35,130 | 1,888
Fair Value, Twelve Months or Longer | 99,447 | 103,336
Fair Value, Total | 134,577 | 105,224
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (40) | (13)
Unrealized Loss, Twelve Months or Longer | (9,321) | (8,908)
Unrealized Loss, Total | (9,361) | (8,921)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Twelve Months or Longer | 22,893 | 23,711
Fair Value, Total | 22,893 | 23,711
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Twelve Months or Longer | (8,044) | (7,664)
Unrealized Loss, Total | (8,044) | (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 | 31,299 | 10,492
Fair Value, Twelve Months or Longer | 81,171 | 80,082
Fair Value, Total | 112,470 | 90,574
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (489) | (222)
Unrealized Loss, Twelve Months or Longer | (7,619) | (7,311)
Unrealized Loss, Total | (8,108) | (7,533)
Corporate Debt Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 204,656 | 132,513
Fair Value, Twelve Months or Longer | 1,500,739 | 1,495,167
Fair Value, Total | 1,705,395 | 1,627,680
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (1,729) | (1,258)
Unrealized Loss, Twelve Months or Longer | (163,286) | (151,541)
Unrealized Loss, Total | (165,015) | (152,799)
Collateralized Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 10,340 | 3,008
Fair Value, Twelve Months or Longer | 242,491 | 248,423
Fair Value, Total | 252,831 | 251,431
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (43) | (23)
Unrealized Loss, Twelve Months or Longer | (44,477) | (45,684)
Unrealized Loss, Total | $ (44,520) | $ (45,707)

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