CSS - auto width floated element (expandable float)

I have two floated collumns side by side. The user can hide/collapse one of that collumns. In that case I want the other collumn to expand to fit the entire container.

Is this possible with CSS?

In resume, it's possible to make a float to expand to the size of it's container? Even if the element is floated, if it has width:auto it should expand. At least that´s way I think it should work.