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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2020			$ 34,652
Foreign currency translation	$ 290	$ 629	(2,627)	$ 3,611
Unrealized net gain on investments	90,839	41,305	45,340	82,093
Change in fair value of cash flow hedges	35	(633)	(497)	(1,534)
Amount reclassified from AOCI	724	(277)	1,288	(321)
Other comprehensive income (loss)	91,888	41,024	43,504	83,849
Balance as of September 30, 2020	78,156		78,156
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			(2,627)
Unrealized net gain on investments			0
Change in fair value of cash flow hedges			0
Amount reclassified from AOCI			0
Other comprehensive income (loss)			(2,627)
Balance as of September 30, 2020	(49,862)		(49,862)
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			45,340
Change in fair value of cash flow hedges			0
Amount reclassified from AOCI			0
Other comprehensive income (loss)			45,340
Balance as of September 30, 2020	136,024		136,024
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			(497)
Amount reclassified from AOCI			1,288
Other comprehensive income (loss)			791
Balance as of September 30, 2020	(5,405)		(5,405)
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 September 30, 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	290	629	(2,627)	3,611
Unrealized net gain on investments	90,839	41,305	45,340	82,093
Change in fair value of cash flow hedges	35	(633)	(497)	(1,534)
Amount reclassified from AOCI	724	$ (277)	1,288	$ (321)
Other comprehensive income (loss)			43,504
Balance as of September 30, 2020	$ 78,156		$ 78,156