Accumulated other comprehensive income (loss) components of net of tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020	$ 34,652
Foreign currency translation	(2,917)	$ 2,982
Unrealized net gain on investments	(45,499)	40,788
Change in fair value of cash flow hedges	(532)	(901)
Amounts reclassified into earnings on hedging activities	564	(44)
Other comprehensive income (loss)	(48,384)	42,825
Balance as of June 30, 2020	(13,732)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020	(47,235)
Foreign currency translation	(2,917)
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)	(2,917)
Balance as of June 30, 2020	(50,152)
Unrealized Net Gain on Investments [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020	90,684
Foreign currency translation	0
Unrealized net gain on investments	(45,499)
Change in fair value of cash flow hedges	0
Amounts reclassified into earnings on hedging activities	0
Other comprehensive income (loss)	(45,499)
Balance as of June 30, 2020	45,185
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	(6,196)
Foreign currency translation	0
Unrealized net gain on investments	0
Change in fair value of cash flow hedges	(532)
Amounts reclassified into earnings on hedging activities	564
Other comprehensive income (loss)	32
Balance as of June 30, 2020	(6,164)
Postretirement Benefit Obligation Gain [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020	(2,601)
Foreign currency translation	0	0
Unrealized net gain on investments	0	0
Change in fair value of cash flow hedges	0	0
Amounts reclassified into earnings on hedging activities	0	0
Other comprehensive income (loss)	0
Balance as of June 30, 2020	(2,601)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020	34,652
Foreign currency translation	(2,917)	2,982
Unrealized net gain on investments	(45,499)	40,788
Change in fair value of cash flow hedges	(532)	(901)
Amounts reclassified into earnings on hedging activities	564	$ (44)
Other comprehensive income (loss)	(48,384)
Balance as of June 30, 2020	$ (13,732)