Stockholders' Equity - Schedule of Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	$ 8,287	$ 10,057	$ 11,273
Other comprehensive income (loss) before reclassifications	53	(207)
Amounts reclassified to earnings and other	96	9
Other Comprehensive Income (Loss), Net of Tax	149	(198)	609
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	3	(66)
Other comprehensive income (loss) attributable to McKesson	146	(132)	424
Ending balance	5,309	8,287	10,057
Foreign Currency Translation Adjustments, Net of Tax
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	(1,628)	(1,258)
Other comprehensive income (loss) before reclassifications	(151)	(431)	804
Amounts reclassified to earnings and other	0	0	0
Other Comprehensive Income (Loss), Net of Tax	(151)	(431)	804
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	1	(61)
Other comprehensive income (loss) attributable to McKesson	(152)	(370)
Ending balance	(1,780)	(1,628)	(1,258)
Unrealized Gains (Losses) on Net Investment Hedges, Net of Tax
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	53	(188)
Other comprehensive income (loss) before reclassifications	85	241	(180)
Amounts reclassified to earnings and other	0	0	0
Other Comprehensive Income (Loss), Net of Tax	85	241	(180)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0	0
Other comprehensive income (loss) attributable to McKesson	85	241
Ending balance	138	53	(188)
Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	(37)	(61)
Other comprehensive income (loss) before reclassifications	86	24	(30)
Amounts reclassified to earnings and other	0	0	0
Other Comprehensive Income (Loss), Net of Tax	86	24	(30)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0	0
Other comprehensive income (loss) attributable to McKesson	86	24
Ending balance	49	(37)	(61)
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	(237)	(210)
Other comprehensive income (loss) before reclassifications	33	(41)
Amounts reclassified to earnings and other	96	9
Other Comprehensive Income (Loss), Net of Tax	129	(32)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	2	(5)
Other comprehensive income (loss) attributable to McKesson	127	(27)
Ending balance	(110)	(237)	(210)
Total Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	(1,849)	(1,717)	(2,141)
Other comprehensive income (loss) attributable to McKesson	146	(132)	424
Ending balance	$ (1,703)	$ (1,849)	$ (1,717)