The 1973 oil crisis strengthened Sweden's commitment to decrease dependence on imported fossil fuels. Since then, electricity has been generated mostly from hydropower and nuclear power. The use of nuclear power has been limited, however. Among other things, the accident of Three Mile Island Nuclear Generating Station (United States) prompted the Riksdag to ban new nuclear plants. In March 2005, an opinion poll showed that 83% supported maintaining or increasing nuclear power. Politicians have made announcements about oil phase-out in Sweden, decrease of nuclear power, and multibillion-dollar investments in renewable energy and energy efficiency. The country has for many years pursued a strategy of indirect taxation as an instrument of environmental policy, including energy taxes in general and carbon dioxide taxes in particular. Sweden was in 2014 a net exporter of electricity by a margin of 16 TWh; the production from windpower mills had increased to 11.5 TWh.