Taxes on Income	3 Months Ended
Mar. 31, 2019
Income Tax Disclosure [Abstract]
Taxes on Income	Taxes on Income The effective income tax rates of 6.7% and 44.9% for the first quarter of 2019 and 2018, 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 rate for the first quarter of 2019 reflects the favorable impact of a $360 million net tax benefit related to the settlement of certain federal income tax matters (discussed below). The effective income tax rate for the first quarter of 2018 reflects the unfavorable impact of a $1.4 billion pretax charge recorded in connection with the formation of a collaboration with Eisai for which no tax benefit was recognized. In the first quarter of 2019, the Internal Revenue Service (IRS) concluded its examinations of Merck’s 2012-2014 U.S. federal income tax returns. As a result, the Company was required to make a payment of $107 million. 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 $360 million net tax benefit in the first quarter of 2019. 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.

Taxes on Income

The effective income tax rates of 6.7% and 44.9% for the first quarter of 2019 and 2018, 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 rate for the first quarter of 2019 reflects the favorable impact of a $360 million net tax benefit related to the settlement of certain federal income tax matters (discussed below). The effective income tax rate for the first quarter of 2018 reflects the unfavorable impact of a $1.4 billion pretax charge recorded in connection with the formation of a collaboration with Eisai for which no tax benefit was recognized.

In the first quarter of 2019, the Internal Revenue Service (IRS) concluded its examinations of Merck’s 2012-2014 U.S. federal income tax returns. As a result, the Company was required to make a payment of $107 million. 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 $360 million net tax benefit in the first quarter of 2019. 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.