Taxes on Income	9 Months Ended
Sep. 30, 2018
Income Tax Disclosure [Abstract]
Taxes on Income	Taxes on Income The effective income tax rates of 26.5% and 125.5% for the third quarter of 2018 and 2017, respectively, and 27.6% and 25.5% for the first nine months of 2018 and 2017, respectively, reflect the impacts of acquisition and divestiture-related costs and restructuring costs, partially offset by the beneficial impact of foreign earnings. In addition, the effective income tax rates for the third quarter and first nine months of 2018 reflect the unfavorable impact of a $420 million aggregate pretax charge related to the termination of a collaboration agreement with Samsung for which no tax benefit was recognized. The effective income tax rate for the first nine months of 2018 also reflects the unfavorable impact of a $1.4 billion aggregate pretax charge recorded in connection with the formation of an oncology collaboration with Eisai for which no tax benefit was recognized. In addition, the effective income tax rates for the third quarter and first nine months of 2017 reflect the unfavorable impact of a $2.35 billion aggregate pretax charge recorded in connection with the formation of an oncology collaboration with AstraZeneca for which no tax benefit was recognized, partially offset by the favorable impact of a net tax benefit of $234 million related to the settlement of certain federal income tax issues (discussed below). The effective income tax rate for the first nine months of 2017 also reflects a benefit of $88 million related to the settlement of a state income tax issue. In the third quarter of 2017, the Internal Revenue Service (IRS) concluded its examinations of Merck’s 2006-2011 U.S. federal income tax returns. As a result, the Company was required to make a payment of approximately $2.8 billion. The Company’s reserves for unrecognized tax benefits for the years under examination exceeded the adjustments relating to this examination period and therefore the Company recorded a net $234 million tax benefit in the third quarter of 2017. This net benefit reflects reductions in reserves for unrecognized tax benefits for tax positions relating to the years that were under examination, partially offset by additional reserves for tax positions not previously reserved for, as well as adjustments to reserves for unrecognized tax benefits relating to years which remain open to examination that are affected by this settlement. On December 22, 2017, new U.S. tax legislation known as the Tax Cuts and Jobs Act of 2017 (TCJA) was enacted. Among other provisions, the TCJA reduced the U.S. federal corporate statutory tax rate from 35% to 21% effective January 1, 2018, requires companies to pay a one-time transition tax on undistributed earnings of certain foreign subsidiaries, and creates new taxes on certain foreign sourced earnings. The Company reflected the impact of the TCJA in its 2017 financial statements as described below. However, application of certain provisions of the TCJA was and remains subject to further interpretation and in these instances the Company made a reasonable estimate of the effects of the TCJA. Changes to these amounts in the first nine months of 2018 were immaterial. The one-time transition tax is based on the Company’s post-1986 undistributed earnings and profits (E&P). For a substantial portion of these undistributed E&P, the Company had not previously provided deferred taxes as these earnings were deemed by Merck to be retained indefinitely by subsidiary companies for reinvestment. The Company recorded a provisional amount for its one-time transition tax liability of $5.3 billion in 2017. Merck has not yet finalized its calculation of the total post-1986 undistributed E&P for these foreign subsidiaries. The transition tax is based in part on the amount of undistributed E&P held in cash and other specified assets; therefore, this amount may change when the Company finalizes its calculation of post-1986 undistributed foreign E&P and finalizes the amounts held in cash or other specified assets. This provisional amount was reduced by the reversal of $2.0 billion of deferred taxes that were previously recorded in connection with the merger of Schering-Plough Corporation in 2009 for certain undistributed foreign E&P. The Company anticipates that it will be able to utilize certain foreign tax credits to partially reduce the transition tax payment, resulting in a net transition tax payment of $5.1 billion. The Company remeasured its deferred tax assets and liabilities at the new federal statutory tax rate of 21%, which resulted in a provisional deferred tax benefit of $779 million in 2017. The deferred tax benefit calculation remains subject to certain clarifications, particularly related to executive compensation and benefits. Beginning in 2018, the TCJA includes a tax on “global intangible low-taxed income” (GILTI) as defined in the TCJA. The Company is allowed to make an accounting policy election to account for the tax effects of the GILTI tax either in the income tax provision in future periods as the tax arises, or as a component of deferred taxes on the related investments in foreign subsidiaries. The Company is currently evaluating the GILTI provisions of the TCJA and the implications on its tax provision and has not finalized the accounting policy election; therefore, the Company has not recorded deferred taxes for GILTI.

Taxes on Income

The effective income tax rates of 26.5% and 125.5% for the third quarter of 2018 and 2017, respectively, and 27.6% and 25.5% for the first nine months of 2018 and 2017, respectively, reflect the impacts of acquisition and divestiture-related costs and restructuring costs, partially offset by the beneficial impact of foreign earnings. In addition, the effective income tax rates for the third quarter and first nine months of 2018 reflect the unfavorable impact of a $420 million aggregate pretax charge related to the termination of a collaboration agreement with Samsung for which no tax benefit was recognized. The effective income tax rate for the first nine months of 2018 also reflects the unfavorable impact of a $1.4 billion aggregate pretax charge recorded in connection with the formation of an oncology collaboration with Eisai for which no tax benefit was recognized. In addition, the effective income tax rates for the third quarter and first nine months of 2017 reflect the unfavorable impact of a $2.35 billion aggregate pretax charge recorded in connection with the formation of an oncology collaboration with AstraZeneca for which no tax benefit was recognized, partially offset by the favorable impact of a net tax benefit of $234 million related to the settlement of certain federal income tax issues (discussed below). The effective income tax rate for the first nine months of 2017 also reflects a benefit of $88 million related to the settlement of a state income tax issue.

In the third quarter of 2017, the Internal Revenue Service (IRS) concluded its examinations of Merck’s 2006-2011 U.S. federal income tax returns. As a result, the Company was required to make a payment of approximately $2.8 billion. The Company’s reserves for unrecognized tax benefits for the years under examination exceeded the adjustments relating to this examination period and therefore the Company recorded a net $234 million tax benefit in the third quarter of 2017. This net benefit reflects reductions in reserves for unrecognized tax benefits for tax positions relating to the years that were under examination, partially offset by additional reserves for tax positions not previously reserved for, as well as adjustments to reserves for unrecognized tax benefits relating to years which remain open to examination that are affected by this settlement.

On December 22, 2017, new U.S. tax legislation known as the Tax Cuts and Jobs Act of 2017 (TCJA) was enacted. Among other provisions, the TCJA reduced the U.S. federal corporate statutory tax rate from 35% to 21% effective January 1, 2018, requires companies to pay a one-time transition tax on undistributed earnings of certain foreign subsidiaries, and creates new taxes on certain foreign sourced earnings. The Company reflected the impact of the TCJA in its 2017 financial statements as described below. However, application of certain provisions of the TCJA was and remains subject to further interpretation and in these instances the Company made a reasonable estimate of the effects of the TCJA. Changes to these amounts in the first nine months of 2018 were immaterial.

The one-time transition tax is based on the Company’s post-1986 undistributed earnings and profits (E&P). For a substantial portion of these undistributed E&P, the Company had not previously provided deferred taxes as these earnings were deemed by Merck to be retained indefinitely by subsidiary companies for reinvestment. The Company recorded a provisional amount for its one-time transition tax liability of $5.3 billion in 2017. Merck has not yet finalized its calculation of the total post-1986 undistributed E&P for these foreign subsidiaries. The transition tax is based in part on the amount of undistributed E&P held in cash and other specified assets; therefore, this amount may change when the Company finalizes its calculation of post-1986 undistributed foreign E&P and finalizes the amounts held in cash or other specified assets. This provisional amount was reduced by the reversal of $2.0 billion of deferred taxes that were previously recorded in connection with the merger of Schering-Plough Corporation in 2009 for certain undistributed foreign E&P. The Company anticipates that it will be able to utilize certain foreign tax credits to partially reduce the transition tax payment, resulting in a net transition tax payment of $5.1 billion.

The Company remeasured its deferred tax assets and liabilities at the new federal statutory tax rate of 21%, which resulted in a provisional deferred tax benefit of $779 million in 2017. The deferred tax benefit calculation remains subject to certain clarifications, particularly related to executive compensation and benefits.

Beginning in 2018, the TCJA includes a tax on “global intangible low-taxed income” (GILTI) as defined in the TCJA. The Company is allowed to make an accounting policy election to account for the tax effects of the GILTI tax either in the income tax provision in future periods as the tax arises, or as a component of deferred taxes on the related investments in foreign subsidiaries. The Company is currently evaluating the GILTI provisions of the TCJA and the implications on its tax provision and has not finalized the accounting policy election; therefore, the Company has not recorded deferred taxes for GILTI.