Comprehensive Income (Loss) (Accumulated other comprehensive income (loss) components of net of tax) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020			$ 46,384
Foreign currency translation	$ (739)	$ 992	(542)	$ (2,400)
Unrealized net gain on investments	(108,444)	41,382	(244,826)	(31,645)
Change in fair value of cash flow hedges	6,289	(56)	6,417	(107)
Amount reclassified from AOCI	19	757	446	1,502
Other comprehensive income (loss)	(102,875)	43,075	(238,505)	(32,650)
Balance as of September 30, 2020	(192,121)		(192,121)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020			(55,757)
Foreign currency translation			(542)
Unrealized net gain on investments			0
Change in fair value of cash flow hedges			0
Amount reclassified from AOCI			0
Other comprehensive income (loss)			(542)
Balance as of September 30, 2020	(56,299)		(56,299)
Unrealized Net Gain on Investments [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020			105,027
Foreign currency translation			0
Unrealized net gain on investments			(244,826)
Change in fair value of cash flow hedges			0
Amount reclassified from AOCI			0
Other comprehensive income (loss)			(244,826)
Balance as of September 30, 2020	(139,799)		(139,799)
Fair Market Value of Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020			(444)
Foreign currency translation			0
Unrealized net gain on investments			0
Change in fair value of cash flow hedges			6,417
Amount reclassified from AOCI			446
Other comprehensive income (loss)			6,863
Balance as of September 30, 2020	6,419		6,419
Postretirement Benefit Obligation Gain [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020			(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 September 30, 2020	(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, 2020			46,384
Foreign currency translation	(739)	992	(542)	(2,400)
Unrealized net gain on investments	(108,444)	41,382	(244,826)	(31,645)
Change in fair value of cash flow hedges	6,289	(56)	6,417	(107)
Amount reclassified from AOCI	19	$ 757	446	$ 1,502
Other comprehensive income (loss)			(238,505)
Balance as of September 30, 2020	$ (192,121)		$ (192,121)