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)
Source: US. Bureau of Economic Analysis
Release: Gross Domestic Product
US. Bureau of Economic Analysis, Real Imports of Goods and Services, GNP-Basis (Discontinued Series) [IMPGSC1N], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/IMPGSC1N/, July 31, 2015.