Google AdSense cuts into sidebar in Safari

I’m trying to add a banner ad to the top of each single post on my website (http://www.margaretandzach.com). When I do so, it looks fine in Firefox, but causes the sidebar to move into the post in Safari and Explorer. Any idea how I can fix this problem (it doesn’t occur on the home page)?

I tried to add a half-banner as a stop-gap measure. Again, it looks fine in Firefox. But in Safari it appears on top of the sidebar–not the post–and a second, truncated add link appears in the box as well.

Thanks!