Accumulated other comprehensive income (loss) components of net of tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2022	Jun. 30, 2021
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2021	$ 46,384
Foreign currency translation	197	$ (3,392)
Unrealized net gain on investments	(136,382)	(73,027)
Change in fair value of cash flow hedges	128	(51)
Amounts reclassified into earnings on hedging activities	427	745
Other comprehensive income (loss)	(135,630)	(75,725)
Balance as of June 30, 2021	(89,246)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2021	(55,757)
Foreign currency translation	197
Unrealized net gain on investments	0
Change in fair value of cash flow hedges	0
Amounts reclassified into earnings on hedging activities	0
Other comprehensive income (loss)	197
Balance as of June 30, 2021	(55,560)
Unrealized Net Gain on Investments [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2021	105,027
Foreign currency translation	0
Unrealized net gain on investments	(136,382)
Change in fair value of cash flow hedges	0
Amounts reclassified into earnings on hedging activities	0
Other comprehensive income (loss)	(136,382)
Balance as of June 30, 2021	(31,355)
Fair Market Value of Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2021	(444)
Foreign currency translation	0
Unrealized net gain on investments	0
Change in fair value of cash flow hedges	128
Amounts reclassified into earnings on hedging activities	427
Other comprehensive income (loss)	555
Balance as of June 30, 2021	111
Postretirement Benefit Obligation Gain [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2021	(2,442)
Foreign currency translation	0
Unrealized net gain on investments	0
Change in fair value of cash flow hedges	0
Amounts reclassified into earnings on hedging activities	0
Other comprehensive income (loss)	0
Balance as of June 30, 2021	(2,442)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2021	46,384
Foreign currency translation	197	(3,392)
Unrealized net gain on investments	(136,382)	(73,027)
Change in fair value of cash flow hedges	128	(51)
Amounts reclassified into earnings on hedging activities	427	$ 745
Other comprehensive income (loss)	(135,630)
Balance as of June 30, 2021	$ (89,246)