CSS Display Table Width Overflow

CSS Display Table Width Overflow

I have two elements on my page placed together in columns like articles on
a newspaper page. One element is an <aside> tag, with the main content
inside of a <section> tag.
The container has a 15px margin to its left that causes it to spill over
the right side of the viewport when its width is set to 100%. How can I
prevent it from doing just that.
You can see the example from this fiddle:
http://jsfiddle.net/spryno724/BHr5F/2/
Note: I know I can use the calc() function to accomplish this task, but
given its current browser support, and my audience, I'm not ready to rely
on this function.