It is estimated that more than half of all US currency is held outside of the US. US currency has historically remained stable and not been devalued, so the demand for US Dollars (USD) tends to go up during times of financial strife in other countries. For example, the total circulation of US currency abroad had some of its highest demand increases during the 1990s and increased byan average of more than 7% per year. This was thought to have been the result of political events worldwide, including the fall of the Berlin Wall and the collapse of the Soviet Union. Historically, $100 USD bills tend to be the most in demand internationally, with an estimated two-thirds of all $100 USD bills being circulated outside the US.
More about US currency: