Source: U.S. Bureau of Economic Analysis
Release: Gross Domestic Product
A Guide to the National Income and Product Accounts of the United States (NIPA) - (http://www.bea.gov/national/pdf/nipaguid.pdf)
Imports (from table 1.1, line 17) plus payments of factor income (From table 1.9, line 3)
U.S. Bureau of Economic Analysis, Imports of Goods and Services, GNP-Basis (DISCONTINUED) [IMPGSN], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/IMPGSN, September 24, 2018.