Investments - (Available-for-sale investments with unrealized losses) (Details) - External Credit Rating, Investment Grade [Member] - USD ($) $ in Thousands	Jun. 30, 2024	Mar. 31, 2024
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	$ 281,425	$ 147,901
Fair Value, Twelve Months or Longer	1,946,741	1,950,719
Fair Value, Total	2,228,166	2,098,620
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(2,301)	(1,516)
Unrealized Loss, Twelve Months or Longer	(232,747)	(221,108)
Unrealized Loss, Total	(235,048)	(222,624)
US Treasury, Government and Corporate Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	35,130	1,888
Fair Value, Twelve Months or Longer	99,447	103,336
Fair Value, Total	134,577	105,224
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(40)	(13)
Unrealized Loss, Twelve Months or Longer	(9,321)	(8,908)
Unrealized Loss, Total	(9,361)	(8,921)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Twelve Months or Longer	22,893	23,711
Fair Value, Total	22,893	23,711
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Twelve Months or Longer	(8,044)	(7,664)
Unrealized Loss, Total	(8,044)	(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	31,299	10,492
Fair Value, Twelve Months or Longer	81,171	80,082
Fair Value, Total	112,470	90,574
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(489)	(222)
Unrealized Loss, Twelve Months or Longer	(7,619)	(7,311)
Unrealized Loss, Total	(8,108)	(7,533)
Corporate Debt Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	204,656	132,513
Fair Value, Twelve Months or Longer	1,500,739	1,495,167
Fair Value, Total	1,705,395	1,627,680
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(1,729)	(1,258)
Unrealized Loss, Twelve Months or Longer	(163,286)	(151,541)
Unrealized Loss, Total	(165,015)	(152,799)
Collateralized Mortgage Backed Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	10,340	3,008
Fair Value, Twelve Months or Longer	242,491	248,423
Fair Value, Total	252,831	251,431
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(43)	(23)
Unrealized Loss, Twelve Months or Longer	(44,477)	(45,684)
Unrealized Loss, Total	$ (44,520)	$ (45,707)