Earnings Per Common Share - Schedule of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Earnings Per Share [Abstract]
Income from continuing operations	$ (1,087)	$ 960	$ 56	$ 363	$ 3,630	$ 649	$ 325	$ 673	$ 292	$ 5,277	$ 2,342
Net income attributable to noncontrolling interests									(230)	(83)	(52)
Income from continuing operations attributable to McKesson									62	5,194	2,290
Income (Loss) from discontinued operations, net of tax	2	1	0	2	(7)	(3)	(1)	(113)	5	(124)	(32)
Net Income Attributable to McKesson Corporation	$ (1,146)	$ 903	$ 1	$ 309	$ 3,588	$ 633	$ 307	$ 542	$ 67	$ 5,070	$ 2,258
Weighted average common shares outstanding:
Basic (in shares)									208	221	230
Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Diluted (in shares)									209	223	233
Diluted
Continuing operations (in dollars per share)	$ (5.58)	$ 4.32	$ 0.01	$ 1.44	$ 16.79	$ 2.86	$ 1.35	$ 2.88	$ 0.30	$ 23.28	$ 9.84
Discontinued operations (in dollars per share)	0.00	0.01	0.00	0.01	(0.03)	(0.01)	(0.01)	(0.50)	0.02	(0.55)	(0.14)
Total (in dollars per share)	(5.58)	4.33	0.01	1.45	16.76	2.85	1.34	2.38	0.32	22.73	9.70
Basic
Continuing operations (in dollars per share)	(5.58)	4.34	0.01	1.46	16.95	2.89	1.36	2.91	0.30	23.50	9.96
Discontinued operations (in dollars per share)	0.00	0.01	0.00	0.00	(0.03)	(0.02)	0.00	(0.50)	0.02	(0.55)	(0.14)
Total (in dollars per share)	$ (5.58)	$ 4.35	$ 0.01	$ 1.46	$ 16.92	$ 2.87	$ 1.36	$ 2.41	$ 0.32	$ 22.95	$ 9.82
Stock Option
Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Effect of dilutive securities (in shares)									0	1	1
Restricted stock units
Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Effect of dilutive securities (in shares)									1	1	2

No definition available.

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- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 55
-Paragraph 52
-URI http://asc.fasb.org/extlink&oid=96948231&loc=d3e4984-109258

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(19))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=96947427&loc=d3e1252-109256

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(21))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(23))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882