A very nice exposition is given in I.G. Macdonalds [Spherical functions on a group of p-adic type](http://www.amazon.com/Spherical-functions-group-p-adic-type/dp/B007EQFY8Q/ref=sr_1_1?ie=UTF8&qid=1453731686&sr=8-1&keywords=spherical+functions+on+the+groups+of+macdonald)