Stockholders' Equity - Schedule of Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 5,653	$ 8,068	$ 5,309	$ 8,287
Other comprehensive income (loss) before reclassifications	13	(23)	40	41
Amounts reclassified to earnings and other	0	79	2	92
Other comprehensive income (loss)	13	56	42	133
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(125)	(18)	(64)	(12)
Other comprehensive income (loss) attributable to McKesson	138	74	106	145
Ending balance	6,090	6,692	6,090	6,692
Foreign Currency Translation Adjustments, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,742)	(1,564)	(1,780)	(1,628)
Other comprehensive income (loss) before reclassifications	111	(114)	207	(44)
Amounts reclassified to earnings and other	0	0	0	0
Other comprehensive income (loss)	111	(114)	207	(44)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(119)	(19)	(61)	(13)
Other comprehensive income (loss) attributable to McKesson	230	(95)	268	(31)
Ending balance	(1,512)	(1,659)	(1,512)	(1,659)
Unrealized Gains on Net Investment Hedges, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	75	27	138	53
Other comprehensive income (loss) before reclassifications	(70)	82	(133)	56
Amounts reclassified to earnings and other	0	0	0	0
Other comprehensive income (loss)	(70)	82	(133)	56
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(1)	0	(1)	0
Other comprehensive income (loss) attributable to McKesson	(69)	82	(132)	56
Ending balance	6	109	6	109
Unrealized gains (losses) on cash flow hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	44	(25)	49	(37)
Other comprehensive income (loss) before reclassifications	(19)	13	(24)	25
Amounts reclassified to earnings and other	0	0	0	0
Other comprehensive income (loss)	(19)	13	(24)	25
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0	0	0	0
Other comprehensive income (loss) attributable to McKesson	(19)	13	(24)	25
Ending balance	25	(12)	25	(12)
Unrealized Net Gains (Losses) and Other Components of Benefit Plans, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(112)	(216)	(110)	(237)
Other comprehensive income (loss) before reclassifications	(9)	(4)	(10)	4
Amounts reclassified to earnings and other	0	79	2	92
Other comprehensive income (loss)	(9)	75	(8)	96
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(5)	1	(2)	1
Other comprehensive income (loss) attributable to McKesson	(4)	74	(6)	95
Ending balance	(116)	(142)	(116)	(142)
Total Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,735)	(1,778)	(1,703)	(1,849)
Other comprehensive income (loss) attributable to McKesson	138	74	106	145
Ending balance	$ (1,597)	$ (1,704)	$ (1,597)	$ (1,704)