Accumulated other comprehensive income (loss) components of net of tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2021	$ 106,857
Foreign currency translation	(3,392)	$ (2,917)
Unrealized net gain on investments	(73,027)	(45,499)
Change in fair value of cash flow hedges	(51)	(532)
Amounts reclassified into earnings on hedging activities	745	564
Other comprehensive income (loss)	(75,725)	(48,384)
Balance as of June 30, 2021	31,132
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2021	(52,929)
Foreign currency translation	(3,392)
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)	(3,392)
Balance as of June 30, 2021	(56,321)
Unrealized Net Gain on Investments [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2021	167,653
Foreign currency translation	0
Unrealized net gain on investments	(73,027)
Change in fair value of cash flow hedges	0
Amounts reclassified into earnings on hedging activities	0
Other comprehensive income (loss)	(73,027)
Balance as of June 30, 2021	94,626
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	(3,879)
Foreign currency translation	0
Unrealized net gain on investments	0
Change in fair value of cash flow hedges	(51)
Amounts reclassified into earnings on hedging activities	745
Other comprehensive income (loss)	694
Balance as of June 30, 2021	(3,185)
Postretirement Benefit Obligation Gain [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2021	(3,988)
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	(3,988)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2021	106,857
Foreign currency translation	(3,392)	(2,917)
Unrealized net gain on investments	(73,027)	(45,499)
Change in fair value of cash flow hedges	(51)	(532)
Amounts reclassified into earnings on hedging activities	745	$ 564
Other comprehensive income (loss)	(75,725)
Balance as of June 30, 2021	$ 31,132