Accumulated other comprehensive income (loss) components of net of tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2023	Jun. 30, 2022
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2023	$ (285,623)
Foreign currency translation	469	$ 197
Unrealized net gain on investments	20,344	(136,382)
Change in fair value of cash flow hedges	3,842	128
Amounts reclassified into earnings on hedging activities	(868)	427
Other comprehensive income (loss)	23,787	(135,630)
Balance as of June 30, 2023	(261,836)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2023	(56,539)
Foreign currency translation	469
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)	469
Balance as of June 30, 2023	(56,070)
Unrealized Net Gain on Investments [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2023	(232,740)
Foreign currency translation	0
Unrealized net gain on investments	20,344
Change in fair value of cash flow hedges	0
Amounts reclassified into earnings on hedging activities	0
Other comprehensive income (loss)	20,344
Balance as of June 30, 2023	(212,396)
Fair Market Value of Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2023	4,007
Foreign currency translation	0
Unrealized net gain on investments	0
Change in fair value of cash flow hedges	3,842
Amounts reclassified into earnings on hedging activities	(868)
Other comprehensive income (loss)	2,974
Balance as of June 30, 2023	6,981
Postretirement Benefit Obligation Gain [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2023	(351)
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, 2023	(351)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2023	(285,623)
Foreign currency translation	469	197
Unrealized net gain on investments	20,344	(136,382)
Change in fair value of cash flow hedges	3,842	128
Amounts reclassified into earnings on hedging activities	(868)	$ 427
Other comprehensive income (loss)	23,787
Balance as of June 30, 2023	$ (261,836)