CSS For Centering Posts

Hello,

I currently have the following CSS code in my custom stylesheet:

.type-post, .type-page {
	background: #cccccc;
	width: 1100px;
	padding: 18px;
	border: 18px solid #a6a6a6;
}

This has gotten my posts to look exactly as I want them, except I need them to be centered on the page. Is there one more line I can add to this code (i.e. “position: ____”, etc) that will center the entire box which contains the posts?
My site is: http://soundsoftheaveragelife.com/ you can easily see what I mean when you check out the site.

Thanks!