For more information, please see the Import/Export Price Indexes frequently asked questions site at http://www.bls.gov/mxp/ippfaq.htm.
Source: US. Bureau of Labor Statistics
Release: U.S. Import and Export Price Indexes
US. Bureau of Labor Statistics, Import (End Use): Gas-natural [IR10110], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/IR10110/, July 29, 2015.