Accumulated Other Comprehensive Income Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020	$ 34,652
Foreign currency translation	(5,694)	$ 9,377	$ (1,759)
Unrealized net loss on investments	76,969	97,943	(59,768)
Change in fair value of cash flow hedges	(429)	(6,301)	451
Amounts reclassified into earnings on hedging activities	2,746	(2)	26
Change in postretirement benfit obligations	(1,387)	333	(1,025)
Other comprehensive income (loss)	72,205	101,350	(62,075)
Balance as of March 31, 2021	106,857	34,652
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020	(47,235)
Foreign currency translation	(5,694)
Unrealized net loss on investments	0
Change in fair value of cash flow hedges	0
Amounts reclassified into earnings on hedging activities	0
Change in postretirement benfit obligations	0
Other comprehensive income (loss)	(5,694)
Balance as of March 31, 2021	(52,929)	(47,235)
Unrealized Gain (Loss) on Investments
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 loss on investments	76,969
Change in fair value of cash flow hedges	0
Amounts reclassified into earnings on hedging activities	0
Change in postretirement benfit obligations	0
Other comprehensive income (loss)	76,969
Balance as of March 31, 2021	167,653	90,684
Fair Market Value of Cash Flow Hedges
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 loss on investments	0
Change in fair value of cash flow hedges	(429)
Amounts reclassified into earnings on hedging activities	2,746
Change in postretirement benfit obligations	0
Other comprehensive income (loss)	2,317
Balance as of March 31, 2021	(3,879)	(6,196)
Postretirement Benefit Obligation Gain (Loss)
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020	(2,601)
Foreign currency translation	0
Unrealized net loss on investments	0
Change in fair value of cash flow hedges	0
Amounts reclassified into earnings on hedging activities	0
Change in postretirement benfit obligations	(1,387)
Other comprehensive income (loss)	(1,387)
Balance as of March 31, 2021	(3,988)	(2,601)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020	34,652
Foreign currency translation	(5,694)	9,377	(1,759)
Unrealized net loss on investments	76,969
Change in fair value of cash flow hedges	(429)
Amounts reclassified into earnings on hedging activities	2,746	(2)	26
Change in postretirement benfit obligations	(1,387)	333	$ (1,025)
Other comprehensive income (loss)	72,205
Balance as of March 31, 2021	$ 106,857	$ 34,652