Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ (19)	$ (16)
Other comprehensive income (loss) before reclassifications	(25)	25
Amounts reclassified from accumulated other comprehensive income (loss)	(27)	(6)
Total other comprehensive income (loss), net of tax	(52)	19
Ending balance	(71)	3
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(3)	1
Other comprehensive income (loss) before reclassifications	0	2
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Total other comprehensive income (loss), net of tax	0	2
Ending balance	(3)	3
Unrealized Gains (Losses) on Derivative Instruments [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	32	14
Other comprehensive income (loss) before reclassifications	(39)	28
Amounts reclassified from accumulated other comprehensive income (loss)	(17)	(6)
Total other comprehensive income (loss), net of tax	(56)	22
Ending balance	(24)	36
Foreign Currency Translation Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(48)	(31)
Other comprehensive income (loss) before reclassifications	14	(5)
Amounts reclassified from accumulated other comprehensive income (loss)	(10)	0
Total other comprehensive income (loss), net of tax	4	(5)
Ending balance	$ (44)	$ (36)