Stockholders' Equity - Schedule of Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	$ 5,309	$ 8,287	$ 10,057
Other comprehensive income (loss) before reclassifications	99	53
Amounts reclassified to earnings and other	71	96
Other comprehensive income (loss), net of tax	170	149	(198)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(53)	3
Other comprehensive income (loss) attributable to McKesson	223	146	(132)
Ending balance	175	5,309	8,287
Foreign Currency Translation Adjustments, Net of Tax
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	(1,780)	(1,628)
Other comprehensive income (loss) before reclassifications	312	(151)	(431)
Amounts reclassified to earnings and other	47	0	0
Other comprehensive income (loss), net of tax	359	(151)	(431)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(60)	1
Other comprehensive income (loss) attributable to McKesson	419	(152)
Ending balance	(1,361)	(1,780)	(1,628)
Unrealized Gains (Losses) on Net Investment Hedges, Net of Tax
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	138	53
Other comprehensive income (loss) before reclassifications	(175)	85	241
Amounts reclassified to earnings and other	0	0	0
Other comprehensive income (loss), net of tax	(175)	85	241
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(1)	0
Other comprehensive income (loss) attributable to McKesson	(174)	85
Ending balance	(36)	138	53
Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	49	(37)
Other comprehensive income (loss) before reclassifications	(36)	86	24
Amounts reclassified to earnings and other	0	0	0
Other comprehensive income (loss), net of tax	(36)	86	24
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0	0
Other comprehensive income (loss) attributable to McKesson	(36)	86
Ending balance	13	49	(37)
Unrealized Net Gains (Losses) and Other Components of Benefit Plans, Net of Tax
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	(110)	(237)
Other comprehensive income (loss) before reclassifications	(2)	33
Amounts reclassified to earnings and other	24	96
Other comprehensive income (loss), net of tax	22	129
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	8	2
Other comprehensive income (loss) attributable to McKesson	14	127
Ending balance	(96)	(110)	(237)
Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	(1,703)	(1,849)	(1,717)
Other comprehensive income (loss) attributable to McKesson	223	146	(132)
Ending balance	$ (1,480)	$ (1,703)	$ (1,849)