re: In 2015, top 20% of income earners paid 83.9% of all federal income taxes

"Why is the share of income taxes negative for 40% of Americans? In recent decades Congress has chosen to funnel important benefits for lower-income earners through the income tax rather than other channels. Some of these benefits, such as the Earned Income Tax Credit and the American Opportunity Credit for education, make cash payments to people who don't owe income tax.

People receiving such payments do pay other federal taxes, of course, such as those for Social Security and Medicare. If these taxes are included, the share of federal taxes paid by the lowest two quintiles turns positive.

The share of tax paid by the top 20% of Americans also changes when such social-insurance levies are included: It drops from more than 80% of income taxes to about 67% of all federal taxes."

From another article on the same data (over at Fox Business)