Investments - (Available-for-sale investments with unrealized losses) (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Mar. 31, 2023
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months	$ 302,654
External Credit Rating, Investment Grade [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Fair Value, Less than Twelve Months		$ 2,089,857
Fair Value, Twelve Months or Longer	2,229,639	523,278
Fair Value, Total	2,532,293	2,613,135
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(13,248)	(183,498)
Unrealized Loss, Twelve Months or Longer	(330,163)	(117,270)
Unrealized Loss, Total	(343,411)	(300,768)
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	6,825	79,846
Fair Value, Twelve Months or Longer	122,372	44,374
Fair Value, Total	129,197	124,220
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(66)	(3,935)
Unrealized Loss, Twelve Months or Longer	(12,957)	(7,639)
Unrealized Loss, Total	(13,023)	(11,574)
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	6,009	2,246
Fair Value, Twelve Months or Longer	31,756	30,670
Fair Value, Total	37,765	32,916
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(52)	(228)
Unrealized Loss, Twelve Months or Longer	(8,301)	(6,707)
Unrealized Loss, Total	(8,353)	(6,935)
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	59,216	108,192
Fair Value, Twelve Months or Longer	82,845	17,079
Fair Value, Total	142,061	125,271
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(3,367)	(8,090)
Unrealized Loss, Twelve Months or Longer	(12,693)	(4,014)
Unrealized Loss, Total	(16,060)	(12,104)
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	213,617	1,697,611
Fair Value, Twelve Months or Longer	1,682,378	272,256
Fair Value, Total	1,895,995	1,969,867
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(9,277)	(158,038)
Unrealized Loss, Twelve Months or Longer	(238,206)	(58,551)
Unrealized Loss, Total	(247,483)	(216,589)
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	16,987	201,962
Fair Value, Twelve Months or Longer	310,288	158,899
Fair Value, Total	327,275	360,861
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less than Twelve Months	(486)	(13,207)
Unrealized Loss, Twelve Months or Longer	(58,006)	(40,359)
Unrealized Loss, Total	$ (58,492)	$ (53,566)