When I set a padding for div .detalhes, the .detalhes box expand over the div .seta. Take a look at:
http://jsfiddle.net/XdfCp/
What can I do to resolve this? My detalhes div has witdh of 100%. I want it consider the padding to calculate the width needed.
Remove the width, and don’t float the div. http://jsfiddle.net/XdfCp/3/
If you’re learning CSS, take some time to understand how floats work, and try to avoid them when there are alternatives. Do not just float an element to “fix” something, it may have nasty consequences.