Accumulated Other Comprehensive Loss (Components of AOCI) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Increase (Decrease) in AOCI [Roll Forward]
Accumulated other comprehensive income (loss), beginning of period	$ (500)	$ (561)	$ (91)
Other comprehensive (loss) income before reclassifications	(117)	43	(494)
Amounts reclassified from AOCI, net	72	18	24
Net current-period other comprehensive (loss) income	(45)	61	(470)
Accumulated other comprehensive income (loss), end of period	(545)	(500)	(561)
Currency Translation [Member]
Increase (Decrease) in AOCI [Roll Forward]
Accumulated other comprehensive income (loss), beginning of period	(196)	(213)	89
Other comprehensive (loss) income before reclassifications	(76)	17	(302)
Amounts reclassified from AOCI, net	16	0	0
Net current-period other comprehensive (loss) income	(60)	17	(302)
Accumulated other comprehensive income (loss), end of period	(256)	(196)	(213)
Cash Flow Hedges [Member]
Increase (Decrease) in AOCI [Roll Forward]
Accumulated other comprehensive income (loss), beginning of period	(11)	(63)	(20)
Other comprehensive (loss) income before reclassifications	(63)	41	(74)
Amounts reclassified from AOCI, net	28	11	31
Net current-period other comprehensive (loss) income	(35)	52	(43)
Accumulated other comprehensive income (loss), end of period	(46)	(11)	(63)
Postretirement Benefit Plans [Member]
Increase (Decrease) in AOCI [Roll Forward]
Accumulated other comprehensive income (loss), beginning of period	(293)	(285)	(160)
Other comprehensive (loss) income before reclassifications	22	(15)	(118)
Amounts reclassified from AOCI, net	28	7	(7)
Net current-period other comprehensive (loss) income	50	(8)	(125)
Accumulated other comprehensive income (loss), end of period	$ (243)	$ (293)	$ (285)