Comprehensive Income (Loss) (Accumulated other comprehensive income (loss) components of net of tax) (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2018	$ (4,623)
Foreign currency translation	(2,093)	$ 8,267
Unrealized net gain on investments	(39,672)	6,911
Change in fair value of cash flow hedges	422	960
Amount reclassified from AOCI	500	$ 1,550
Balance as of June 30, 2018	(45,966)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2018	(54,853)
Foreign currency translation	(2,093)
Unrealized net gain on investments
Change in fair value of cash flow hedges
Amount reclassified from AOCI
Other comprehensive income (loss)	(2,093)
Balance as of June 30, 2018	(56,946)
Unrealized Net Gain on Investments [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2018	52,509
Foreign currency translation
Unrealized net gain on investments	(39,672)
Change in fair value of cash flow hedges
Amount reclassified from AOCI
Other comprehensive income (loss)	(39,672)
Balance as of June 30, 2018	12,837
Fair Market Value of Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2018	(370)
Foreign currency translation
Unrealized net gain on investments
Change in fair value of cash flow hedges	922
Amount reclassified from AOCI	(500)
Other comprehensive income (loss)	422
Balance as of June 30, 2018	52
Postretirement Benefit Obligation Gain [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2018	(1,909)
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 June 30, 2018	(1,909)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) Components, Net of Tax [Roll Forward]
Balance as of March 31, 2018	(4,623)
Foreign currency translation	(2,093)
Unrealized net gain on investments	(39,672)
Change in fair value of cash flow hedges	922
Amount reclassified from AOCI	(500)
Other comprehensive income (loss)	(41,343)
Balance as of June 30, 2018	$ (45,966)