CSS for line break after highlight box in post

Hi,
I’ve added a CSS class to the Suffusion theme so I can use a highlight box in my post content. The box looks great but I can’t figure out how to add a line break after it. Adding a bottom margin didn’t work.

Here is the css:

.post .highlightbox {
	background-color: #444444;
	border: 1px solid #888;
	padding-left: 4px;
	padding-right: 4px;
	float: left;
	margin: 0 8px 0 8px;
}

I can’t get a line break after the <div> or the <p> in the post html so the text following the box just continues on the same line.