Stockholders' Equity - Schedule of Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 10,057
Other comprehensive income (loss) before reclassifications	$ (76)		(169)
Amounts reclassified to earnings	1		5
Other Comprehensive Income (Loss), Net of Tax	(75)	$ (4)	(164)	$ 576
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(11)		(55)
Other comprehensive income (loss) attributable to McKesson	(64)		(109)
Ending balance	9,389		9,389
Foreign Currency Translation Adjustments, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,480)		(1,258)
Other comprehensive income (loss) before reclassifications	(188)	30	(456)	715
Amounts reclassified to earnings	0	0	0	0
Other Comprehensive Income (Loss), Net of Tax	(188)	30	(456)	715
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(11)		(57)
Other comprehensive income (loss) attributable to McKesson	(177)		(399)
Ending balance	(1,657)		(1,657)
Unrealized Gains (Losses) on Net Investment Hedges, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(23)		(188)
Other comprehensive income (loss) before reclassifications	75	(19)	240	(127)
Amounts reclassified to earnings	0	0	0	0
Other Comprehensive Income (Loss), Net of Tax	75	(19)	240	(127)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0		0
Other comprehensive income (loss) attributable to McKesson	75		240
Ending balance	52		52
Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(59)		(61)
Other comprehensive income (loss) before reclassifications	35	(16)	37	(5)
Amounts reclassified to earnings	0	0	0	0
Other Comprehensive Income (Loss), Net of Tax	35	$ (16)	37	$ (5)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0		0
Other comprehensive income (loss) attributable to McKesson	35		37
Ending balance	(24)		(24)
Unrealized Net Gains (Losses) and Other Components of Benefit Plans, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(200)		(210)
Other comprehensive income (loss) before reclassifications	2		10
Amounts reclassified to earnings	1		5
Other Comprehensive Income (Loss), Net of Tax	3		15
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0		2
Other comprehensive income (loss) attributable to McKesson	3		13
Ending balance	(197)		(197)
Total Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,762)		(1,717)
Ending balance	$ (1,826)		$ (1,826)