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, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2019			$ (66,698)
Foreign currency translation	$ (692)	$ (4,101)	2,919	$ (5,081)
Unrealized net gain on investments	22,378		104,471
Change in fair value of cash flow hedges	361	57		528
Amount reclassified from AOCI			(2)	23
Balance as of June 30, 2019	39,517		39,517
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2019			(56,612)
Foreign currency translation			2,919
Unrealized net gain on investments			0
Change in fair value of cash flow hedges			0
Amount reclassified from AOCI			0
Other comprehensive income (loss)			2,919
Balance as of June 30, 2019	(53,693)		(53,693)
Unrealized Net Gain on Investments [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2019			(7,259)
Foreign currency translation			0
Unrealized net gain on investments			104,471
Change in fair value of cash flow hedges			0
Amount reclassified from AOCI			0
Other comprehensive income (loss)			104,471
Balance as of June 30, 2019	97,212		97,212
Fair Market Value of Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2019			107
Foreign currency translation			0
Unrealized net gain on investments			0
Change in fair value of cash flow hedges			(1,173)
Amount reclassified from AOCI			(2)
Other comprehensive income (loss)			(1,175)
Balance as of June 30, 2019	(1,068)		(1,068)
Postretirement Benefit Obligation Gain [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2019			(2,934)
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 June 30, 2019	(2,934)		(2,934)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2019			(66,698)
Foreign currency translation	(692)	(4,101)	2,919	(5,081)
Unrealized net gain on investments	22,378	(8,281)	104,471	(55,830)
Change in fair value of cash flow hedges	361	57	(1,173)	528
Amount reclassified from AOCI	319	$ (144)	(2)	$ 23
Other comprehensive income (loss)			106,215
Balance as of June 30, 2019	$ 39,517		$ 39,517