Stockholders' Equity - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]
Balance	$ 11,725	$ 13,837
Current-period other comprehensive loss	(180)	(439)
Amounts reclassified from accumulated other comprehensive income (loss)	(4)	(1)
Balance	8,870	11,837
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]
Balance	(517)	(261)
Current-period other comprehensive loss	(184)	(408)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Balance	(701)	(669)
Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]
Balance	(3)	9
Current-period other comprehensive loss	2	(30)
Amounts reclassified from accumulated other comprehensive income (loss)	0	5
Balance	(1)	(16)
Available-for-sale Securities
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]
Balance	9	9
Current-period other comprehensive loss	2	(1)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Balance	11	8
Pension and Other Post-retirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]
Balance	267	301
Current-period other comprehensive loss	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	(4)	(6)
Balance	263	295
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]
Balance	(244)	58
Balance	$ (428)	$ (382)