Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ (71)	$ 3	$ (19)	[1]	$ (16)
Other comprehensive income (loss) before reclassifications	3	(8)	(22)		17
Amounts reclassified from accumulated other comprehensive income (loss)	(5)	(5)	(32)		(11)
Total other comprehensive income (loss), net of tax	(2)	(13)	(54)		6
Ending balance	(73)	(10)	(73)		(10)
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(3)	3	(3)		1
Other comprehensive income (loss) before reclassifications	0	(1)	0		1
Amounts reclassified from accumulated other comprehensive income (loss)	0	(1)	0		(1)
Total other comprehensive income (loss), net of tax	0	(2)	0		0
Ending balance	(3)	1	(3)		1
Unrealized Gains (Losses) on Derivative Instruments [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(24)	36	32		14
Other comprehensive income (loss) before reclassifications	(29)	(1)	(68)		27
Amounts reclassified from accumulated other comprehensive income (loss)	(5)	(4)	(22)		(10)
Total other comprehensive income (loss), net of tax	(34)	(5)	(90)		17
Ending balance	(58)	31	(58)		31
Foreign Currency Translation Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(44)	(36)	(48)		(31)
Other comprehensive income (loss) before reclassifications	32	(6)	46		(11)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	(10)		0
Total other comprehensive income (loss), net of tax	32	(6)	36		(11)
Ending balance	$ (12)	$ (42)	$ (12)		$ (42)

[1] Derived from audited Consolidated Financial Statements.