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, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 11,273
Other comprehensive income (loss) before reclassifications	$ (5)		573
Amounts reclassified to earnings and other	1		3
Other Comprehensive Income (Loss), Net of Tax	(4)	$ (404)	576	$ (762)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	12		161
Other comprehensive income (loss) attributable to McKesson	(16)		415
Ending balance	11,972		11,972
Foreign Currency Translation Adjustments, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,336)		(1,873)
Other comprehensive income (loss) before reclassifications	30	(398)	715	(782)
Amounts reclassified to earnings and other	0	0	0	20
Other Comprehensive Income (Loss), Net of Tax	30	(398)	715	(762)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	12		160
Other comprehensive income (loss) attributable to McKesson	18		555
Ending balance	(1,318)		(1,318)
Unrealized Gains (Losses) on Net Investment Hedges, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(116)		(8)
Other comprehensive income (loss) before reclassifications	(19)	0	(127)	0
Amounts reclassified to earnings and other	0	0	0	0
Other Comprehensive Income (Loss), Net of Tax	(19)	0	(127)	0
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0		0
Other comprehensive income (loss) attributable to McKesson	(19)		(127)
Ending balance	(135)		(135)
Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(20)		(31)
Other comprehensive income (loss) before reclassifications	(16)		(5)
Amounts reclassified to earnings and other	0		0
Other Comprehensive Income (Loss), Net of Tax	(16)	$ (14)	(5)	$ (20)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0		0
Other comprehensive income (loss) attributable to McKesson	(16)		(5)
Ending balance	(36)		(36)
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(238)		(229)
Other comprehensive income (loss) before reclassifications	0		(10)
Amounts reclassified to earnings and other	1		3
Other Comprehensive Income (Loss), Net of Tax	1		(7)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0		1
Other comprehensive income (loss) attributable to McKesson	1		(8)
Ending balance	(237)		(237)
Total Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,710)		(2,141)
Ending balance	$ (1,726)		$ (1,726)