Stockholders' Equity - Schedule of Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	$ 11,273	$ 9,008	$ 8,085
Other comprehensive income (loss) before reclassifications	604	(688)
Amounts reclassified to earnings	5	29
Other Comprehensive Income (Loss), Net of Tax	609	(659)	172
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(185)	79
Other comprehensive income (loss) attributable to McKesson	424	(580)	152
Ending balance	10,057	11,273	9,008
Foreign Currency Translation Adjustments, Net of Tax
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	(1,873)	(1,323)
Other comprehensive income (loss) before reclassifications	804	(644)	113
Amounts reclassified to earnings	0	20	0
Other Comprehensive Income (Loss), Net of Tax	804	(624)	113
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(189)	74
Other comprehensive income (loss) attributable to McKesson	615	(550)
Ending balance	(1,258)	(1,873)	(1,323)
Unrealized Losses on Net Investment Hedges, Net of Tax
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	(8)	0
Other comprehensive income (loss) before reclassifications	(180)	(8)	0
Amounts reclassified to earnings	0	0	0
Other Comprehensive Income (Loss), Net of Tax	(180)	(8)	0
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0	0
Other comprehensive income (loss) attributable to McKesson	(180)	(8)
Ending balance	(188)	(8)	0
Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	(31)	(12)
Other comprehensive income (loss) before reclassifications	(30)	(19)	6
Amounts reclassified to earnings	0	0	3
Other Comprehensive Income (Loss), Net of Tax	(30)	(19)	9
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0	0
Other comprehensive income (loss) attributable to McKesson	(30)	(19)
Ending balance	(61)	(31)	(12)
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	(229)	(226)
Other comprehensive income (loss) before reclassifications	10	(17)
Amounts reclassified to earnings	5	9
Other Comprehensive Income (Loss), Net of Tax	15	(8)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	4	5
Other comprehensive income (loss) attributable to McKesson	19	(3)
Ending balance	(210)	(229)	(226)
Total Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	(2,141)	(1,561)	(1,713)
Other comprehensive income (loss) attributable to McKesson	424	(580)	152
Ending balance	$ (1,717)	$ (2,141)	$ (1,561)