Investments - (Available-for-sale investments with unrealized losses) (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Mar. 31, 2025
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	$ 432,615
Fair Value, Twelve Months or Longer	1,591,378
Fair Value, Total	2,023,993
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(5,534)
Unrealized Loss, Twelve Months or Longer	(189,169)
Unrealized Loss, Total	(194,703)
External Credit Rating, Investment Grade [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months		$ 567,128
Fair Value, Twelve Months or Longer		1,625,793
Fair Value, Total		2,192,921
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months		(11,582)
Unrealized Loss, Twelve Months or Longer		(217,293)
Unrealized Loss, Total		(228,875)
US Treasury, Government and Corporate Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Total	97,116
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	1,459	1,760
Fair Value, Twelve Months or Longer	95,657	95,058
Fair Value, Total		96,818
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(2)	(24)
Unrealized Loss, Twelve Months or Longer	(6,627)	(8,329)
Unrealized Loss, Total	(6,629)	(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	22,220
Fair Value, Total	42,968
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(143)
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		36,871
Fair Value, Twelve Months or Longer	20,748	20,928
Fair Value, Total		57,799
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months		(197)
Unrealized Loss, Twelve Months or Longer	(8,205)	(8,515)
Unrealized Loss, Total	(8,348)	(8,712)
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Total	95,439
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	41,243	46,036
Fair Value, Twelve Months or Longer	54,196	52,903
Fair Value, Total		98,939
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(1,530)	(1,628)
Unrealized Loss, Twelve Months or Longer	(6,729)	(7,179)
Unrealized Loss, Total	(8,259)	(8,807)
Corporate Debt Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Total	1,439,687
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	229,829	294,133
Fair Value, Twelve Months or Longer	1,209,858	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	(2,724)	(5,822)
Unrealized Loss, Twelve Months or Longer	(130,475)	(149,927)
Unrealized Loss, Total	(133,199)	(155,749)
Collateralized Mortgage Backed Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Total	348,783
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	137,864	188,328
Fair Value, Twelve Months or Longer	210,919	217,020
Fair Value, Total		405,348
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(1,135)	(3,911)
Unrealized Loss, Twelve Months or Longer	(37,133)	(43,343)
Unrealized Loss, Total	$ (38,268)	$ (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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