Stockholders' Equity (Deficit) - Schedule of Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 6,090	$ 6,692	$ 5,309	$ 8,287
Other comprehensive income (loss) before reclassifications	50	45	90	86
Amounts reclassified to earnings and other	69	6	71	98
Other comprehensive income (loss)	119	51	161	184
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	25	10	(39)	(2)
Other comprehensive income (loss) attributable to McKesson	94	41	200	186
Ending balance	(277)	6,385	(277)	6,385
Total Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,597)	(1,704)	(1,703)	(1,849)
Other comprehensive income (loss) attributable to McKesson	94	41	200	186
Ending balance	(1,503)	(1,663)	(1,503)	(1,663)
Foreign Currency Translation Adjustments, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,512)	(1,659)	(1,780)	(1,628)
Other comprehensive income (loss) before reclassifications	156	101	363	57
Amounts reclassified to earnings and other	47	0	47	0
Other comprehensive income (loss)	203	101	410	57
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	20	12	(41)	(1)
Other comprehensive income (loss) attributable to McKesson	183	89	451	58
Ending balance	(1,329)	(1,570)	(1,329)	(1,570)
Unrealized Gains (Losses) on Net Investment Hedges, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	6	109	138	53
Other comprehensive income (loss) before reclassifications	(96)	(58)	(229)	(2)
Amounts reclassified to earnings and other	0	0	0	0
Other comprehensive income (loss)	(96)	(58)	(229)	(2)
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0	0	(1)	0
Other comprehensive income (loss) attributable to McKesson	(96)	(58)	(228)	(2)
Ending balance	(90)	51	(90)	51
Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	25	(12)	49	(37)
Other comprehensive income (loss) before reclassifications	(12)	8	(36)	33
Amounts reclassified to earnings and other	0	0	0	0
Other comprehensive income (loss)	(12)	8	(36)	33
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	0	0	0	0
Other comprehensive income (loss) attributable to McKesson	(12)	8	(36)	33
Ending balance	13	(4)	13	(4)
Unrealized Net Losses and Other Components of Benefit Plans, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(116)	(142)	(110)	(237)
Other comprehensive income (loss) before reclassifications	2	(6)	(8)	(2)
Amounts reclassified to earnings and other	22	6	24	98
Other comprehensive income (loss)	24	0	16	96
Less: amounts attributable to noncontrolling and redeemable noncontrolling interests	5	(2)	3	(1)
Other comprehensive income (loss) attributable to McKesson	19	2	13	97
Ending balance	$ (97)	$ (140)	$ (97)	$ (140)