Comprehensive Income (Loss) (Accumulated other comprehensive income (loss) components of net of tax) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020			$ 106,857
Foreign currency translation	$ 992	$ 290	(2,400)	$ (2,627)
Unrealized net gain on investments	41,382	90,839	(31,645)	45,340
Change in fair value of cash flow hedges	(56)	35	(107)	(497)
Amount reclassified from AOCI	757	724	1,502	1,288
Other comprehensive income (loss)	43,075	91,888	(32,650)	43,504
Balance as of September 30, 2020	74,207		74,207
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020			(52,929)
Foreign currency translation			(2,400)
Unrealized net gain on investments			0
Change in fair value of cash flow hedges			0
Amount reclassified from AOCI			0
Other comprehensive income (loss)			(2,400)
Balance as of September 30, 2020	(55,329)		(55,329)
Unrealized Net Gain on Investments [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020			167,653
Foreign currency translation			0
Unrealized net gain on investments			(31,645)
Change in fair value of cash flow hedges			0
Amount reclassified from AOCI			0
Other comprehensive income (loss)			(31,645)
Balance as of September 30, 2020	136,008		136,008
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			(3,879)
Foreign currency translation			0
Unrealized net gain on investments			0
Change in fair value of cash flow hedges			(107)
Amount reclassified from AOCI			1,502
Other comprehensive income (loss)			1,395
Balance as of September 30, 2020	(2,484)		(2,484)
Postretirement Benefit Obligation Gain [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020			(3,988)
Foreign currency translation			0
Unrealized net gain on investments			0
Change in fair value of cash flow hedges			0
Amount reclassified from AOCI			0
Other comprehensive income (loss)			0
Balance as of September 30, 2020	(3,988)		(3,988)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020			106,857
Foreign currency translation	992	290	(2,400)	(2,627)
Unrealized net gain on investments	41,382	90,839	(31,645)	45,340
Change in fair value of cash flow hedges	(56)	35	(107)	(497)
Amount reclassified from AOCI	757	$ 724	1,502	$ 1,288
Other comprehensive income (loss)			(32,650)
Balance as of September 30, 2020	$ 74,207		$ 74,207