Comprehensive Income (Loss) (Accumulated other comprehensive income (loss) components of net of tax) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2022	Dec. 31, 2021
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2022			$ 46,384
Foreign currency translation	$ (215)	$ 112	(757)	$ (2,288)
Unrealized net gain on investments	(91,550)	(11,992)	(336,376)	(43,637)
Change in fair value of cash flow hedges	115	178	6,532	71
Amount reclassified from AOCI	(345)	759	101	2,261
Other comprehensive income (loss)	(91,995)	(10,943)	(330,500)	(43,593)
Balance as of December 31, 2022	(284,116)		(284,116)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2022			(55,757)
Foreign currency translation			(757)
Unrealized net gain on investments			0
Change in fair value of cash flow hedges			0
Amount reclassified from AOCI			0
Other comprehensive income (loss)			(757)
Balance as of December 31, 2022	(56,514)		(56,514)
Unrealized Net Gain on Investments [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2022			105,027
Foreign currency translation			0
Unrealized net gain on investments			(336,376)
Change in fair value of cash flow hedges			0
Amount reclassified from AOCI			0
Other comprehensive income (loss)			(336,376)
Balance as of December 31, 2022	(231,349)		(231,349)
Fair Market Value of Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2022			(444)
Foreign currency translation			0
Unrealized net gain on investments			0
Change in fair value of cash flow hedges			6,532
Amount reclassified from AOCI			101
Other comprehensive income (loss)			6,633
Balance as of December 31, 2022	6,189		6,189
Postretirement Benefit Obligation Gain [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2022			(2,442)
Foreign currency translation			0
Unrealized net gain on investments			0
Change in fair value of cash flow hedges			0
Amount reclassified from AOCI			0
Other comprehensive income (loss)			0
Balance as of December 31, 2022	(2,442)		(2,442)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2022			46,384
Foreign currency translation	(215)	112	(757)	(2,288)
Unrealized net gain on investments	(91,550)	(11,992)	(336,376)	(43,637)
Change in fair value of cash flow hedges	115	178	6,532	71
Amount reclassified from AOCI	(345)	$ 759	101	$ 2,261
Other comprehensive income (loss)			(330,500)
Balance as of December 31, 2022	$ (284,116)		$ (284,116)