Investments - (Available-for-sale investments with unrealized losses) (Details) - External Credit Rating, Investment Grade [Member] - USD ($) $ in Thousands	Sep. 30, 2025	Mar. 31, 2025
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	$ 327,188	$ 567,128
Fair Value, Twelve Months or Longer	1,547,413	1,625,793
Fair Value, Total	1,874,601	2,192,921
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(5,488)	(11,582)
Unrealized Loss, Twelve Months or Longer	(174,225)	(217,293)
Unrealized Loss, Total	(179,713)	(228,875)
US Treasury, Government and Corporate Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months		1,760
Fair Value, Twelve Months or Longer	94,669	95,058
Fair Value, Total	94,669	96,818
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months		(24)
Unrealized Loss, Twelve Months or Longer	(6,012)	(8,329)
Unrealized Loss, Total	(6,012)	(8,353)
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	10,695	36,871
Fair Value, Twelve Months or Longer	19,921	20,928
Fair Value, Total	30,616	57,799
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(46)	(197)
Unrealized Loss, Twelve Months or Longer	(7,486)	(8,515)
Unrealized Loss, Total	(7,532)	(8,712)
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	41,058	46,036
Fair Value, Twelve Months or Longer	53,467	52,903
Fair Value, Total	94,525	98,939
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(1,439)	(1,628)
Unrealized Loss, Twelve Months or Longer	(6,858)	(7,179)
Unrealized Loss, Total	(8,297)	(8,807)
Corporate Debt Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	186,091	294,133
Fair Value, Twelve Months or Longer	1,181,934	1,239,884
Fair Value, Total	1,368,025	1,534,017
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(1,651)	(5,822)
Unrealized Loss, Twelve Months or Longer	(119,051)	(149,927)
Unrealized Loss, Total	(120,702)	(155,749)
Collateralized Mortgage Backed Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	89,344	188,328
Fair Value, Twelve Months or Longer	197,422	217,020
Fair Value, Total	286,766	405,348
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(2,352)	(3,911)
Unrealized Loss, Twelve Months or Longer	(34,818)	(43,343)
Unrealized Loss, Total	$ (37,170)	$ (47,254)

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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