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Investments - (Available-for-sale investments with unrealized losses) (Details) - External Credit Rating, Investment Grade [Member] - USD ($) $ in Thousands | Mar. 31, 2024 | Mar. 31, 2023
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Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | $ 147,901 | $ 1,922,962
Fair Value, Twelve Months or Longer | 1,950,719 | 406,651
Fair Value, Total | 2,098,620 | 2,329,613
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (1,516) | (183,498)
Unrealized Loss, Twelve Months or Longer | (221,108) | (117,270)
Unrealized Loss, Total | (222,624) | (300,768)
US Treasury, Government and Corporate Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 1,888 | 75,952
Fair Value, Twelve Months or Longer | 103,336 | 36,736
Fair Value, Total | 105,224 | 112,688
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (13) | (3,935)
Unrealized Loss, Twelve Months or Longer | (8,908) | (7,639)
Unrealized Loss, Total | (8,921) | (11,574)
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 |  | 2,019
Fair Value, Twelve Months or Longer | 23,711 | 23,966
Fair Value, Total | 23,711 | 25,985
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months |  | (228)
Unrealized Loss, Twelve Months or Longer | (7,664) | (6,707)
Unrealized Loss, Total | (7,664) | (6,935)
US States and Political Subdivisions Debt Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 10,492 | 101,974
Fair Value, Twelve Months or Longer | 80,082 | 13,463
Fair Value, Total | 90,574 | 115,437
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (222) | (8,090)
Unrealized Loss, Twelve Months or Longer | (7,311) | (4,014)
Unrealized Loss, Total | (7,533) | (12,104)
Corporate Debt Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 132,513 | 1,553,647
Fair Value, Twelve Months or Longer | 1,495,167 | 213,947
Fair Value, Total | 1,627,680 | 1,767,594
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (1,258) | (158,038)
Unrealized Loss, Twelve Months or Longer | (151,541) | (58,551)
Unrealized Loss, Total | (152,799) | (216,589)
Collateralized Mortgage-Backed Securities [Member] |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] |  |
Fair Value, Less than Twelve Months | 3,008 | 189,370
Fair Value, Twelve Months or Longer | 248,423 | 118,539
Fair Value, Total | 251,431 | 307,909
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Unrealized Loss, Less than Twelve Months | (23) | (13,207)
Unrealized Loss, Twelve Months or Longer | (45,684) | (40,359)
Unrealized Loss, Total | $ (45,707) | $ (53,566)

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