Investments - (Available-for-sale investments with unrealized losses) (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Mar. 31, 2024
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	$ 344,537
Fair Value, Twelve Months or Longer	1,907,363
Fair Value, Total	2,251,900
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(2,841)
Unrealized Loss, Twelve Months or Longer	(241,185)
Unrealized Loss, Total	(244,026)
External Credit Rating, Investment Grade [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months		$ 147,901
Fair Value, Twelve Months or Longer		1,950,719
Fair Value, Total		2,098,620
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months		(1,516)
Unrealized Loss, Twelve Months or Longer		(221,108)
Unrealized Loss, Total		(222,624)
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	39,648	1,888
Fair Value, Twelve Months or Longer	97,088	103,336
Fair Value, Total	136,736	105,224
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(35)	(13)
Unrealized Loss, Twelve Months or Longer	(9,313)	(8,908)
Unrealized Loss, Total	(9,348)	(8,921)
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, Twelve Months or Longer	22,087	23,711
Fair Value, Total	22,087	23,711
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Twelve Months or Longer	(8,255)	(7,664)
Unrealized Loss, Total	(8,255)	(7,664)
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	37,607	10,492
Fair Value, Twelve Months or Longer	79,068	80,082
Fair Value, Total	116,675	90,574
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(676)	(222)
Unrealized Loss, Twelve Months or Longer	(8,422)	(7,311)
Unrealized Loss, Total	(9,098)	(7,533)
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	208,344	132,513
Fair Value, Twelve Months or Longer	1,475,083	1,495,167
Fair Value, Total	1,683,427	1,627,680
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(1,953)	(1,258)
Unrealized Loss, Twelve Months or Longer	(169,655)	(151,541)
Unrealized Loss, Total	(171,608)	(152,799)
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	58,938	3,008
Fair Value, Twelve Months or Longer	234,037	248,423
Fair Value, Total	292,975	251,431
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(177)	(23)
Unrealized Loss, Twelve Months or Longer	(45,540)	(45,684)
Unrealized Loss, Total	$ (45,717)	$ (45,707)