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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2016			$ (60,525)
Foreign currency translation	$ (1,704)	$ (13,098)	(1,982)	$ (10,565)
Unrealized net gain on investments	20,013	(20,556)	39,401	(13,631)
Change in fair value of cash flow hedges	1,926	766	3,216	2,855
Amount reclassified from AOCI			5,107	6,621
Other comprehensive income (loss)	196,710	$ 150,491	364,283	$ 333,323
Balance as of September 30, 2016	(19,890)		(19,890)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2016			(63,643)
Foreign currency translation			(1,982)
Unrealized net gain on investments
Change in fair value of cash flow hedges
Amount reclassified from AOCI
Other comprehensive income (loss)			(1,982)
Balance as of September 30, 2016	(65,625)		(65,625)
Unrealized Net Gain on Investments [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2016			14,115
Foreign currency translation
Unrealized net gain on investments			39,401
Change in fair value of cash flow hedges
Amount reclassified from AOCI
Other comprehensive income (loss)			39,401
Balance as of September 30, 2016	53,516		53,516
Fair Market Value of Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2016			(9,208)
Foreign currency translation
Unrealized net gain on investments
Change in fair value of cash flow hedges			8,323
Amount reclassified from AOCI			(5,107)
Other comprehensive income (loss)			3,216
Balance as of September 30, 2016	(5,992)		(5,992)
Postretirement Benefit Obligation Gain [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2016			(1,789)
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 September 30, 2016	(1,789)		(1,789)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2016			(60,525)
Foreign currency translation			(1,982)
Unrealized net gain on investments			39,401
Change in fair value of cash flow hedges			8,323
Amount reclassified from AOCI			(5,107)
Other comprehensive income (loss)			40,635
Balance as of September 30, 2016	$ (19,890)		$ (19,890)