Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes On March 27, 2020, the Coronavirus Aid, Relief, and Economic Security Act ("CARES Act") was enacted into law. The primary impact to the Company’s financial statements as a result of the CARES Act was the deferral of US corporate income tax payments from the second quarter of 2020 to July 2020, as well as the deferral of employer related payroll tax payments from the second, third and fourth quarters of 2020 with 50% to be paid in the fourth quarter of 2021 and the remaining 50% to be paid in the fourth quarter of 2022. Income tax expense was $57 million and $59 million for the three months ended September 30, 2020 and 2019, respectively. The effective tax rate, expressed by calculating the income tax expense as a percentage of Income before income taxes, was 22.7% for both the three months ended September 30, 2020 and 2019. The effective tax rates differed from the US statutory rate of 21.0% for the three months ended September 30, 2020 primarily due to state and local income taxes and a discrete deferred tax expense as a result of an increase in the UK corporate tax rate, partially offset by excess tax benefits on equity-based compensation. The effective tax rate differed from the US statutory rate of 21.0% for the three months ended September 30, 2019 primarily due to state and local income taxes, partially offset by excess tax benefits on equity-based compensation. Income tax expense was $157 million and $163 million for the nine months ended September 30, 2020 and 2019, respectively. The effective tax rate, expressed by calculating the income tax expense as a percentage of Income before income taxes, was 22.2% for the nine months ended September 30, 2020 and differed from the US federal statutory rate of 21.0% with state and local income taxes and a discrete deferred tax expense as a result of an increase in the UK corporate tax rate being largely offset by excess tax benefits on equity-based compensation. The effective tax rate for the nine months ended September 30, 2019 was 22.8% and differed from the US federal statutory rate of 21.0% primarily due to state and local income taxes, partially offset by excess tax benefits on equity-based compensation and a discrete tax benefit related to CDW Canada's acquisition of Scalar Decisions Inc. ("Scalar") in 2019.

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.