Data Type to use for Exchange Rate in SSAS Multidimensional

So we have a cube setup that does a many-to-many currency conversion. Until this point, the Exchange Rate measure group has used a Currency data type for the exchange rate, but I didn’t realize that that’s limited to 4 decimal places. With the only other Data Types in SSAS being Double and Single, what are people using for their exchange rate data types when the rates may need to be more then 4 decimal places? How many decimal places would a double be accurate to from a FX rate perspective?