Go to Window > Workspace and reset your current workspace.
Docked panels keep this from happening. When you undock them from the left (Tools panel), top (Control panel) or right (all the other panels) the document window can slide underneath.
Reset your workspace as per Steve, or if you've changed your workspace but haven't saved it yet, just redock them.
Undocked panels over the document window:
Grab the panel title bar (the very top row) and drag the mouse cursor over to the far right. Don't let go until you see the blue vertical line to the right of the scroll bar.
Since both of you are right, how do I share credit for the right answer?
I'd give Steve the correct answer because he was both first and correct, but if you found mine to be helpful, feel free to mark it helpful.