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, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax
Balance as of March 31, 2017			$ (51,236)
Foreign currency translation	$ (1,794)	$ (5,821)	19,240	$ (7,803)
Unrealized net gain on investments	2,852	2,755	19,820	42,156
Change in fair value of cash flow hedges	565	1,922	2,265	5,138
Amount reclassified from AOCI			3,308	6,940
Other comprehensive income (loss)	530,517	$ 64,084	821,065	$ 428,367
Balance as of December 31, 2017	(9,911)		(9,911)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax
Balance as of March 31, 2017			(69,505)
Foreign currency translation			19,240
Unrealized net gain on investments
Change in fair value of cash flow hedges
Amount reclassified from AOCI
Other comprehensive income (loss)			19,240
Balance as of December 31, 2017	(50,265)		(50,265)
Unrealized Net Gain on Investments [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax
Balance as of March 31, 2017			23,099
Foreign currency translation
Unrealized net gain on investments			19,820
Change in fair value of cash flow hedges
Amount reclassified from AOCI
Other comprehensive income (loss)			19,820
Balance as of December 31, 2017	42,919		42,919
Fair Market Value of Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax
Balance as of March 31, 2017			(3,059)
Foreign currency translation
Unrealized net gain on investments
Change in fair value of cash flow hedges			5,573
Amount reclassified from AOCI			(3,308)
Other comprehensive income (loss)			2,265
Balance as of December 31, 2017	(794)		(794)
Postretirement Benefit Obligation Gain [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax
Balance as of March 31, 2017			(1,771)
Foreign currency translation
Unrealized net gain on investments
Change in fair value of cash flow hedges
Amount reclassified from AOCI
Other comprehensive income (loss)
Balance as of December 31, 2017	(1,771)		(1,771)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax
Balance as of March 31, 2017			(51,236)
Foreign currency translation			19,240
Unrealized net gain on investments			19,820
Change in fair value of cash flow hedges			5,573
Amount reclassified from AOCI			(3,308)
Other comprehensive income (loss)			41,325
Balance as of December 31, 2017	$ (9,911)		$ (9,911)