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 1.2, line 17) plus payments of factor income (From table 1.10, line 3)
U.S. Bureau of Economic Analysis, Real Imports of Goods and Services, GNP-Basis (DISCONTINUED) [IMPGSC1N], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/IMPGSC1N, April 21, 2018.