The Uzbek economy is in a gradual transition to the market economy, with foreign trade policy being based on import substitution. In September 2017, the country's currency became fully convertible at market rates. Uzbekistan is a major producer and exporter of cotton. With the gigantic power-generation facilities from the Soviet era and an ample supply of natural gas, Uzbekistan has become the largest electricity producer in Central Asia. As of late 2018, the republic was given a BB- rating by both Standard and Poor (S&P) and Fitch. Strengths indicated by Brookings Institution include Uzbekistan having large liquid assets, high economic growth and low public debt. Among the constraints holding the republic back are low GDP per capita.