This line is much less well-known that the part of the same speech in which Calvin Coolidge says that the main business of American is business. In this part of the speech, Coolidge is trying to say that Americans are good and generous. Earlier in the speech, he said Americans were concerned with business and getting rich. But here, he says that this does not mean that they are selfish and greedy.
Here is the portion of the speech that contains the line:
We make no concealment of the fact that we want wealth, but there are many other things that we want very much more. We want peace and honor, and that charity which is so strong an element of all civilization. The chief ideal of the American people is idealism. I cannot repeat too often that America is a nation of idealists. That is the only motive to which they ever give any strong and lasting reaction.
Coolidge is clearly saying that Americans like wealth but that that is not all they like. Instead, they are also interested in things like peace and honor and charity. This is the sense in which Americans are devoted to idealism.