Stockholders' Equity - Schedule of Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 6,692	$ 9,534	$ 8,287	$ 10,057
Other comprehensive income (loss) before reclassifications	45	(76)	86	(169)
Amounts reclassified to earnings	6	1	98	5
Other Comprehensive Income (Loss), Net of Tax	51	(75)	184	(164)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	10	(11)	(2)	(55)
Other comprehensive income (loss) attributable to McKesson	41	(64)	186	(109)
Ending balance	6,385	9,389	6,385	9,389
Foreign Currency Translation Adjustments, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,659)	(1,480)	(1,628)	(1,258)
Other comprehensive income (loss) before reclassifications	101	(188)	57	(456)
Amounts reclassified to earnings	0	0	0	0
Other Comprehensive Income (Loss), Net of Tax	101	(188)	57	(456)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	12	(11)	(1)	(57)
Other comprehensive income (loss) attributable to McKesson	89	(177)	58	(399)
Ending balance	(1,570)	(1,657)	(1,570)	(1,657)
Unrealized Gains (Losses) on Net Investment Hedges, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	109	(23)	53	(188)
Other comprehensive income (loss) before reclassifications	(58)	75	(2)	240
Amounts reclassified to earnings	0	0	0	0
Other Comprehensive Income (Loss), Net of Tax	(58)	75	(2)	240
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0	0	0	0
Other comprehensive income (loss) attributable to McKesson	(58)	75	(2)	240
Ending balance	51	52	51	52
Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(12)	(59)	(37)	(61)
Other comprehensive income (loss) before reclassifications	8	35	33	37
Amounts reclassified to earnings	0	0	0	0
Other Comprehensive Income (Loss), Net of Tax	8	35	33	37
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0	0	0	0
Other comprehensive income (loss) attributable to McKesson	8	35	33	37
Ending balance	(4)	(24)	(4)	(24)
Unrealized Net Gains (Losses) and Other Components of Benefit Plans, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(142)	(200)	(237)	(210)
Other comprehensive income (loss) before reclassifications	(6)	2	(2)	10
Amounts reclassified to earnings	6	1	98	5
Other Comprehensive Income (Loss), Net of Tax	0	3	96	15
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(2)	0	(1)	2
Other comprehensive income (loss) attributable to McKesson	2	3	97	13
Ending balance	(140)	(197)	(140)	(197)
Total Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,704)	(1,762)	(1,849)	(1,717)
Other comprehensive income (loss) attributable to McKesson	41	(64)	186	(109)
Ending balance	$ (1,663)	$ (1,826)	$ (1,663)	$ (1,826)