Income Taxes	9 Months Ended
Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Our reported income tax benefit rates were 37.7% and 3.5% for the third quarter and first nine months of 2018 compared to income tax expense rates of 16.8% and 25.7% for the third quarter and first nine months of 2017. Fluctuations in our reported income tax rates are primarily due to discrete items mainly driven by the impact of the 2017 Tax Act, as discussed below, the impact of nondeductible impairment charges, changes within our business mix of income, and the effect of an intercompany sale of software. During the third quarters of 2018 and 2017, income tax benefit was $263 million and income tax expense was $131 million related to continuing operations and included net discrete tax benefits of $424 million and $12 million. During the first nine months of 2018 and 2017, income tax benefit was $46 million and tax expense was $570 million related to continuing operations and included net discrete tax benefits of $420 million and $69 million. Our discrete tax benefits for 2018 included a provisional $370 million related to the impact of the 2017 Tax Act, further described below, and other discrete tax benefits of $54 million primarily related to the conclusion of certain tax audits. Our discrete tax benefits for the first nine months of 2017 included $47 million related to the adoption of the amended accounting guidance on employee share-based compensation. The non-cash pre-tax charge of $350 million to impair the carrying value of goodwill related to our McKesson Europe reporting unit within our Distribution Solutions segment, described in our Financial Note 3, “Goodwill Impairment Charges,” had an unfavorable impact on our effective tax rate in 2018 given that this charge was not tax deductible. The non-cash pre-tax charge of $290 million to impair the carrying value of goodwill related to our EIS business within our Technology Solutions segment, described in Financial Note 3, "Goodwill Impairment Charges," had an unfavorable impact on our effective tax rate in 2017 given that approximately $269 million of the goodwill impairment charge was not tax deductible. We signed the Revenue Agent’s Report from the U.S. Internal Revenue Services (“IRS”) relating to 2010 through 2012 on December 29, 2017. We file income tax returns in the U.S. federal jurisdiction, various U.S. state jurisdictions and various foreign jurisdictions. We are subject to audit by the IRS for fiscal years 2013 through the current fiscal year. We are generally subject to audit by taxing authorities in various U.S. states and in foreign jurisdictions for fiscal years 2010 through the current fiscal year. As of December 31, 2017, we had $944 million of unrecognized tax benefits, of which $833 million would reduce income tax expense and the effective tax rate, if recognized. The increase in unrecognized tax benefit is mainly due to uncertainty relating to the application of the 2017 Tax Act, partially offset by the impact of the IRS audit resolution. During the next twelve months, we do not anticipate a significant increase or decrease to our unrecognized tax benefits based on the information currently available. However, this amount may change as we continue to have ongoing negotiations with various taxing authorities throughout the year and complete our accounting related to the impact of the 2017 Tax Act. 2017 Tax Act On December 22, 2017, the U.S. government enacted comprehensive new tax legislation referred to as the 2017 Tax Act. The 2017 Tax Act makes broad and complex changes to the U.S. tax code that affect our fiscal year 2018, including but not limited to, (1) reducing the U.S. federal corporate tax rate from 35 percent to 21 percent; and (2) requiring companies to pay a one-time transition tax on certain unrepatriated earnings of foreign subsidiaries. The 2017 Tax Act also establishes new tax provisions that will affect our fiscal year 2019, including, but not limited to, (1) eliminating the corporate alternative minimum tax (“AMT”); (2) creating the base erosion anti-abuse tax (“BEAT”); (3) establishing new limitations on deductible interest expense and certain executive compensation; (4) creating a new provision designed to tax global intangible low-tax income (“GILTI”); (5) generally eliminating U.S. federal income taxes on dividends from foreign subsidiaries; and (6) changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017. The accounting guidance on income taxes requires us to recognize the effects of new legislation upon enactment. Accordingly, we are required to recognize the effects of the 2017 Tax Act in the third quarter of 2018. Shortly after the enactment, however, the SEC staff issued guidance on accounting for the 2017 Tax Act. This guidance provides a measurement period that should not extend beyond one year from the 2017 Tax Act enactment date for companies to complete the accounting for income taxes. In accordance with the SEC staff guidance, a company must reflect the income tax effects of those aspects of the 2017 Tax Act for which the accounting for the income taxes is complete. To the extent that a company’s accounting for the income tax effect of certain provisions of the 2017 Tax Act is incomplete but the company is able to determine a reasonable estimate, it must record a provisional estimate in the financial statements. If a company cannot determine a provisional estimate to be included in the financial statements, it should continue to apply the accounting guidance on income taxes on the basis of the provisions of the tax laws that were in effect immediately before the enactment of the 2017 Tax Act. Regarding the new GILTI tax rules, we are allowed to make an accounting policy election to either (1) treat taxes due on future GILTI inclusions in U.S. taxable income as a current-period expense when incurred or (2) reflect such portion of the future GILTI inclusions in U.S. taxable income that relate to existing basis differences in the company’s current measurement of deferred taxes. Our analysis of the new GILTI rules and how they may impact us is incomplete. Accordingly, we have not made a policy election regarding the treatment of the GILTI tax. In connection with our initial analysis of the impact of the 2017 Tax Act, we recorded a net discrete tax benefit of $370 million during the third quarter of 2018. This net benefit mainly arises from changing the expected future consequences of settling differences between the book and tax basis of assets and liabilities, mainly driven by a decrease of our deferred tax liabilities for inventories and investments; partially offset by establishing a new obligation for the taxation of certain unrepatriated earnings of our foreign subsidiaries. Although our accounting for the impact of the 2017 Tax Act is incomplete, we have made reasonable estimates and recorded provisional amounts as follows: Reduction of U.S. federal corporate tax rate: The 2017 Tax Act reduces the corporate tax rate from 35 percent to 21 percent, effective January 1, 2018. U.S. tax law stipulates that our fiscal year 2018 will have a blended tax rate of 31.6 percent, which is based on the pro rata number of days in the fiscal year before and after the effective date. For the fiscal year 2019, the tax rate will be 21 percent. As a result, we have remeasured certain deferred tax assets and deferred tax liabilities and recorded a provisional net discrete tax benefit of $1.26 billion, mainly driven by a decrease of our deferred tax liabilities for inventories and investments. While we were able to make a reasonable estimate of the impact of the reduction in the corporate tax rate, it may be affected by, among other items, changes to estimates the company has utilized to calculate the reversal pattern of our existing temporary differences and the state effect of adjustments made to federal temporary differences. Deemed Repatriation Transition Tax (“Transition Tax”): The 2017 Tax Act imposes a Transition Tax on certain accumulated earnings and profits (“E&P”) of our foreign subsidiaries. To determine the amount of the Transition Tax, we must determine, in addition to other factors, the amount of post-1986 E&P of the relevant subsidiaries as well as the amount of non-U.S. income taxes paid on such earnings. We were able to make a reasonable estimate of the impact of the Transition Tax and recorded a provisional discrete tax expense of $434 million. This estimate may change as we gather additional information to more precisely compute the amount of the Transition Tax. Uncertainty relating to the application of the new legislation: The 2017 Tax Act makes broad and complex changes to the U.S. tax code, including substantial changes to the taxation of cumulative foreign earnings and the treatment of future U.S. inclusions. The application of certain provisions of the 2017 Tax Act may involve some uncertainty. Accordingly, we recognized a provisional discrete tax expense of $452 million to increase our unrecognized tax benefits and to reflect the amount of benefit that is more likely than not expected to be sustained. This estimate may change, among other things, due to clarifications of the application of certain provisions of the 2017 Tax Act.

Income Taxes

Our reported income tax benefit rates were 37.7% and 3.5% for the third quarter and first nine months of 2018 compared to income tax expense rates of 16.8% and 25.7% for the third quarter and first nine months of 2017. Fluctuations in our reported income tax rates are primarily due to discrete items mainly driven by the impact of the 2017 Tax Act, as discussed below, the impact of nondeductible impairment charges, changes within our business mix of income, and the effect of an intercompany sale of software.

During the third quarters of 2018 and 2017, income tax benefit was $263 million and income tax expense was $131 million related to continuing operations and included net discrete tax benefits of $424 million and $12 million. During the first nine months of 2018 and 2017, income tax benefit was $46 million and tax expense was $570 million related to continuing operations and included net discrete tax benefits of $420 million and $69 million.

Our discrete tax benefits for 2018 included a provisional $370 million related to the impact of the 2017 Tax Act, further described below, and other discrete tax benefits of $54 million primarily related to the conclusion of certain tax audits. Our discrete tax benefits for the first nine months of 2017 included $47 million related to the adoption of the amended accounting guidance on employee share-based compensation.

The non-cash pre-tax charge of $350 million to impair the carrying value of goodwill related to our McKesson Europe reporting unit within our Distribution Solutions segment, described in our Financial Note 3, “Goodwill Impairment Charges,” had an unfavorable impact on our effective tax rate in 2018 given that this charge was not tax deductible.

The non-cash pre-tax charge of $290 million to impair the carrying value of goodwill related to our EIS business within our Technology Solutions segment, described in Financial Note 3, "Goodwill Impairment Charges," had an unfavorable impact on our effective tax rate in 2017 given that approximately $269 million of the goodwill impairment charge was not tax deductible.

We signed the Revenue Agent’s Report from the U.S. Internal Revenue Services (“IRS”) relating to 2010 through 2012 on December 29, 2017. We file income tax returns in the U.S. federal jurisdiction, various U.S. state jurisdictions and various foreign jurisdictions. We are subject to audit by the IRS for fiscal years 2013 through the current fiscal year. We are generally subject to audit by taxing authorities in various U.S. states and in foreign jurisdictions for fiscal years 2010 through the current fiscal year.

As of December 31, 2017, we had $944 million of unrecognized tax benefits, of which $833 million would reduce income tax expense and the effective tax rate, if recognized. The increase in unrecognized tax benefit is mainly due to uncertainty relating to the application of the 2017 Tax Act, partially offset by the impact of the IRS audit resolution. During the next twelve months, we do not anticipate a significant increase or decrease to our unrecognized tax benefits based on the information currently available. However, this amount may change as we continue to have ongoing negotiations with various taxing authorities throughout the year and complete our accounting related to the impact of the 2017 Tax Act.

2017 Tax Act

On December 22, 2017, the U.S. government enacted comprehensive new tax legislation referred to as the 2017 Tax Act. The 2017 Tax Act makes broad and complex changes to the U.S. tax code that affect our fiscal year 2018, including but not limited to, (1) reducing the U.S. federal corporate tax rate from 35 percent to 21 percent; and (2) requiring companies to pay a one-time transition tax on certain unrepatriated earnings of foreign subsidiaries.

The 2017 Tax Act also establishes new tax provisions that will affect our fiscal year 2019, including, but not limited to, (1) eliminating the corporate alternative minimum tax (“AMT”); (2) creating the base erosion anti-abuse tax (“BEAT”); (3) establishing new limitations on deductible interest expense and certain executive compensation; (4) creating a new provision designed to tax global intangible low-tax income (“GILTI”); (5) generally eliminating U.S. federal income taxes on dividends from foreign subsidiaries; and (6) changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017.

The accounting guidance on income taxes requires us to recognize the effects of new legislation upon enactment. Accordingly, we are required to recognize the effects of the 2017 Tax Act in the third quarter of 2018. Shortly after the enactment, however, the SEC staff issued guidance on accounting for the 2017 Tax Act. This guidance provides a measurement period that should not extend beyond one year from the 2017 Tax Act enactment date for companies to complete the accounting for income taxes. In accordance with the SEC staff guidance, a company must reflect the income tax effects of those aspects of the 2017 Tax Act for which the accounting for the income taxes is complete. To the extent that a company’s accounting for the income tax effect of certain provisions of the 2017 Tax Act is incomplete but the company is able to determine a reasonable estimate, it must record a provisional estimate in the financial statements. If a company cannot determine a provisional estimate to be included in the financial statements, it should continue to apply the accounting guidance on income taxes on the basis of the provisions of the tax laws that were in effect immediately before the enactment of the 2017 Tax Act.

Regarding the new GILTI tax rules, we are allowed to make an accounting policy election to either (1) treat taxes due on future GILTI inclusions in U.S. taxable income as a current-period expense when incurred or (2) reflect such portion of the future GILTI inclusions in U.S. taxable income that relate to existing basis differences in the company’s current measurement of deferred taxes. Our analysis of the new GILTI rules and how they may impact us is incomplete. Accordingly, we have not made a policy election regarding the treatment of the GILTI tax.

In connection with our initial analysis of the impact of the 2017 Tax Act, we recorded a net discrete tax benefit of $370 million during the third quarter of 2018. This net benefit mainly arises from changing the expected future consequences of settling differences between the book and tax basis of assets and liabilities, mainly driven by a decrease of our deferred tax liabilities for inventories and investments; partially offset by establishing a new obligation for the taxation of certain unrepatriated earnings of our foreign subsidiaries. Although our accounting for the impact of the 2017 Tax Act is incomplete, we have made reasonable estimates and recorded provisional amounts as follows:

Reduction of U.S. federal corporate tax rate: The 2017 Tax Act reduces the corporate tax rate from 35 percent to 21 percent, effective January 1, 2018. U.S. tax law stipulates that our fiscal year 2018 will have a blended tax rate of 31.6 percent, which is based on the pro rata number of days in the fiscal year before and after the effective date. For the fiscal year 2019, the tax rate will be 21 percent. As a result, we have remeasured certain deferred tax assets and deferred tax liabilities and recorded a provisional net discrete tax benefit of $1.26 billion, mainly driven by a decrease of our deferred tax liabilities for inventories and investments. While we were able to make a reasonable estimate of the impact of the reduction in the corporate tax rate, it may be affected by, among other items, changes to estimates the company has utilized to calculate the reversal pattern of our existing temporary differences and the state effect of adjustments made to federal temporary differences.

Deemed Repatriation Transition Tax (“Transition Tax”): The 2017 Tax Act imposes a Transition Tax on certain accumulated earnings and profits (“E&P”) of our foreign subsidiaries. To determine the amount of the Transition Tax, we must determine, in addition to other factors, the amount of post-1986 E&P of the relevant subsidiaries as well as the amount of non-U.S. income taxes paid on such earnings. We were able to make a reasonable estimate of the impact of the Transition Tax and recorded a provisional discrete tax expense of $434 million. This estimate may change as we gather additional information to more precisely compute the amount of the Transition Tax.

Uncertainty relating to the application of the new legislation: The 2017 Tax Act makes broad and complex changes to the U.S. tax code, including substantial changes to the taxation of cumulative foreign earnings and the treatment of future U.S. inclusions. The application of certain provisions of the 2017 Tax Act may involve some uncertainty. Accordingly, we recognized a provisional discrete tax expense of $452 million to increase our unrecognized tax benefits and to reflect the amount of benefit that is more likely than not expected to be sustained. This estimate may change, among other things, due to clarifications of the application of certain provisions of the 2017 Tax Act.