Brunei gained its independence from the United Kingdom on 1 January 1984. Economic growth during the 1990s and 2000s, with the GDP increasing 56% from 1999 to 2008, transformed Brunei into an industrialised country. It has developed wealth from extensive petroleum and natural gas fields. Brunei has the second-highest Human Development Index among the Southeast Asian nations, after Singapore, and is classified as a developed country. According to the International Monetary Fund (IMF), Brunei is ranked fifth in the world by gross domestic product per capita at purchasing power parity. The IMF estimated in 2011 that Brunei was one of two countries (the other being Libya) with a public debt at 0% of the national GDP.