footer's padding making it expand over its parent div width

footer's padding making it expand over its parent div width

I have a footer in my webpage with some text in it. When i try to add
padding-left property to it, the width of the footer expands over the
enclosing parent div element and this problem only happens with Opera and
not with Chrome or Firefox or IE.. What should be done to correct it ??
CSS for footer :
footer {
color: #FFFFFF;
background-color: #3399CC;
padding-top: 12px;
padding-bottom: 12px;
width: 100%;
float: left;
}