Investments - (Available-for-sale investments with unrealized losses) (Details) - External Credit Rating, Investment Grade [Member] - USD ($) $ in Thousands	Dec. 31, 2024	Mar. 31, 2024
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	$ 245,220	$ 147,901
Fair Value, Twelve Months or Longer	1,724,691	1,950,719
Fair Value, Total	1,969,911	2,098,620
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(998)	(1,516)
Unrealized Loss, Twelve Months or Longer	(165,747)	(221,108)
Unrealized Loss, Total	(166,745)	(222,624)
US Treasury, Government and Corporate Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months		1,888
Fair Value, Twelve Months or Longer	99,405	103,336
Fair Value, Total	99,405	105,224
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months		(13)
Unrealized Loss, Twelve Months or Longer	(6,094)	(8,908)
Unrealized Loss, Total	(6,094)	(8,921)
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	37,449
Fair Value, Twelve Months or Longer	22,153	23,711
Fair Value, Total	59,602	23,711
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(210)
Unrealized Loss, Twelve Months or Longer	(7,733)	(7,664)
Unrealized Loss, Total	(7,943)	(7,664)
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	23,020	10,492
Fair Value, Twelve Months or Longer	59,752	80,082
Fair Value, Total	82,772	90,574
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(252)	(222)
Unrealized Loss, Twelve Months or Longer	(5,216)	(7,311)
Unrealized Loss, Total	(5,468)	(7,533)
Corporate Debt Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	120,515	132,513
Fair Value, Twelve Months or Longer	1,314,484	1,495,167
Fair Value, Total	1,434,999	1,627,680
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(330)	(1,258)
Unrealized Loss, Twelve Months or Longer	(111,361)	(151,541)
Unrealized Loss, Total	(111,691)	(152,799)
Collateralized Mortgage Backed Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	64,236	3,008
Fair Value, Twelve Months or Longer	228,897	248,423
Fair Value, Total	293,133	251,431
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(206)	(23)
Unrealized Loss, Twelve Months or Longer	(35,343)	(45,684)
Unrealized Loss, Total	$ (35,549)	$ (45,707)