HTML:
<div id="main">
<div id="content"></div>
<div id="sidebar"></div>
</div>
CSS:
#content {
background: blue;
height: 900px;
width: calc(100% - 200px);
}
#sidebar {
float: right;
width: 200px;
height: 900px;
background: red;
}
JSFIDDLE: http://jsfiddle.net/aFrPc/
To achieve the desired layout, I have rearranged the order of the div elements in the HTML code. The #content now comes before #sidebar and has been set to fill all remaining space using the CSS property 'width: calc(100% - 200px);'.
Image of final wanted result: