Investments - (Available-for-sale investments with unrealized losses) (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Mar. 31, 2025
External Credit Rating, Investment Grade [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	$ 163,780	$ 567,128
Fair Value, Twelve Months or Longer	1,483,300	1,625,793
Fair Value, Total	1,647,080	2,192,921
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(3,186)	(11,582)
Unrealized Loss, Twelve Months or Longer	(151,879)	(217,293)
Unrealized Loss, Total	(155,065)	(228,875)
US Treasury, Government and Corporate Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Total	92,760
US Treasury, Government and Corporate Securities [Member] \| External Credit Rating, Investment Grade [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months		1,760
Fair Value, Twelve Months or Longer	92,760	95,058
Fair Value, Total		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	(5,320)	(8,329)
Unrealized Loss, Total	(5,320)	(8,353)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Total	56,838
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] \| External Credit Rating, Investment Grade [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	37,360	36,871
Fair Value, Twelve Months or Longer	19,478	20,928
Fair Value, Total		57,799
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(190)	(197)
Unrealized Loss, Twelve Months or Longer	(7,482)	(8,515)
Unrealized Loss, Total	(7,672)	(8,712)
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Total	80,653
US States and Political Subdivisions Debt Securities [Member] \| External Credit Rating, Investment Grade [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	18,120	46,036
Fair Value, Twelve Months or Longer	62,533	52,903
Fair Value, Total		98,939
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(434)	(1,628)
Unrealized Loss, Twelve Months or Longer	(5,470)	(7,179)
Unrealized Loss, Total	(5,904)	(8,807)
Corporate Debt Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Total	1,186,289
Corporate Debt Securities [Member] \| External Credit Rating, Investment Grade [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	72,656	294,133
Fair Value, Twelve Months or Longer	1,113,633	1,239,884
Fair Value, Total		1,534,017
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(370)	(5,822)
Unrealized Loss, Twelve Months or Longer	(101,965)	(149,927)
Unrealized Loss, Total	(102,335)	(155,749)
Collateralized Mortgage Backed Securities [Member] \| External Credit Rating, Investment Grade [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	35,644	188,328
Fair Value, Twelve Months or Longer	194,896	217,020
Fair Value, Total	230,540	405,348
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(2,192)	(3,911)
Unrealized Loss, Twelve Months or Longer	(31,642)	(43,343)
Unrealized Loss, Total	$ (33,834)	$ (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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