Stockholders' Equity - Schedule of Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 10,057
Other comprehensive income (loss) before reclassifications	(121)
Amounts reclassified to earnings	0
Other Comprehensive Income (Loss), Net of Tax	(121)	$ 321
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(37)
Other comprehensive income (loss) attributable to McKesson	(84)
Ending balance	9,647
Foreign Currency Translation Adjustments, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,258)
Other comprehensive income (loss) before reclassifications	(273)	382
Amounts reclassified to earnings	0	0
Other Comprehensive Income (Loss), Net of Tax	(273)	382
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	(39)
Other comprehensive income (loss) attributable to McKesson	(234)
Ending balance	(1,492)
Unrealized Losses on Net Investment Hedges, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(188)
Other comprehensive income (loss) before reclassifications	144	(70)
Amounts reclassified to earnings	0	0
Other Comprehensive Income (Loss), Net of Tax	144	(70)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0
Other comprehensive income (loss) attributable to McKesson	144
Ending balance	(44)
Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(61)
Other comprehensive income (loss) before reclassifications	0	14
Amounts reclassified to earnings	0	0
Other Comprehensive Income (Loss), Net of Tax	0	$ 14
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0
Other comprehensive income (loss) attributable to McKesson	0
Ending balance	(61)
Unrealized Net Gains (Losses) and Other Components of Benefit Plans, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(210)
Other comprehensive income (loss) before reclassifications	8
Amounts reclassified to earnings	0
Other Comprehensive Income (Loss), Net of Tax	8
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	2
Other comprehensive income (loss) attributable to McKesson	6
Ending balance	(204)
Total Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,717)
Ending balance	$ (1,801)