Comprehensive Income (Loss) (Accumulated other comprehensive income (loss) components of net of tax) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020			$ 34,652
Foreign currency translation	$ (2,566)	$ (692)	(5,193)	$ 2,919
Unrealized net gain on investments	9,560	22,378	54,900	104,471
Change in fair value of cash flow hedges	26	361	(471)	(1,173)
Amount reclassified from AOCI	734	319	2,022	(2)
Other comprehensive income (loss)	7,754	22,366	51,258	106,215
Balance as of December 31, 2020	85,910		85,910
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			(5,193)
Unrealized net gain on investments			0
Change in fair value of cash flow hedges			0
Amount reclassified from AOCI			0
Other comprehensive income (loss)			(5,193)
Balance as of December 31, 2020	(52,428)		(52,428)
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			54,900
Change in fair value of cash flow hedges			0
Amount reclassified from AOCI			0
Other comprehensive income (loss)			54,900
Balance as of December 31, 2020	145,584		145,584
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			(471)
Amount reclassified from AOCI			2,022
Other comprehensive income (loss)			1,551
Balance as of December 31, 2020	(4,645)		(4,645)
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	0	0
Unrealized net gain on investments	0	0	0	0
Change in fair value of cash flow hedges	0	0	0	0
Amount reclassified from AOCI	0	0	0	0
Other comprehensive income (loss)			0
Balance as of December 31, 2020	(2,601)		(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	(2,566)	(692)	(5,193)	2,919
Unrealized net gain on investments	9,560	22,378	54,900	104,471
Change in fair value of cash flow hedges	26	361	(471)	(1,173)
Amount reclassified from AOCI	734	$ 319	2,022	$ (2)
Other comprehensive income (loss)			51,258
Balance as of December 31, 2020	$ 85,910		$ 85,910