Stockholders' Equity - Schedule of Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	$ 10,057	$ 11,273	$ 9,008
Other comprehensive income (loss) before reclassifications	(207)	604
Amounts reclassified to earnings	9	5
Other Comprehensive Income (Loss), Net of Tax	(198)	609	(659)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(66)	185
Other comprehensive income (loss) attributable to McKesson	(132)	424	(580)
Ending balance	8,287	10,057	11,273
Foreign Currency Translation Adjustments, Net of Tax
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	(1,258)	(1,873)
Other comprehensive income (loss) before reclassifications	(431)	804	(644)
Amounts reclassified to earnings	0	0	20
Other Comprehensive Income (Loss), Net of Tax	(431)	804	(624)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(61)	189
Other comprehensive income (loss) attributable to McKesson	(370)	615
Ending balance	(1,628)	(1,258)	(1,873)
Unrealized Losses on Net Investment Hedges, Net of Tax
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	(188)	(8)
Other comprehensive income (loss) before reclassifications	241	(180)	(8)
Amounts reclassified to earnings	0	0	0
Other Comprehensive Income (Loss), Net of Tax	241	(180)	(8)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0	0
Other comprehensive income (loss) attributable to McKesson	241	(180)
Ending balance	53	(188)	(8)
Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	(61)	(31)
Other comprehensive income (loss) before reclassifications	24	(30)	(19)
Amounts reclassified to earnings	0	0	0
Other Comprehensive Income (Loss), Net of Tax	24	(30)	(19)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0	0
Other comprehensive income (loss) attributable to McKesson	24	(30)
Ending balance	(37)	(61)	(31)
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	(210)	(229)
Other comprehensive income (loss) before reclassifications	(41)	10
Amounts reclassified to earnings	9	5
Other Comprehensive Income (Loss), Net of Tax	(32)	15
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(5)	(4)
Other comprehensive income (loss) attributable to McKesson	(27)	19
Ending balance	(237)	(210)	(229)
Total Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Change, Net of Tax [Roll Forward]
Beginning balance	(1,717)	(2,141)	(1,561)
Other comprehensive income (loss) attributable to McKesson	(132)	424	(580)
Ending balance	$ (1,849)	$ (1,717)	$ (2,141)