Investments - (Available-for-sale investments with unrealized losses) (Details) - External Credit Rating, Investment Grade [Member] - USD ($) $ in Thousands	Mar. 31, 2024	Mar. 31, 2023
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)