Over swinging your arms during the backswing is a common mistake.
The source of this fault can be put down to not loading the body correctly, letting the right leg straighten during the backswing, or simply not understanding how the arms should actually move during the backswing.
Without the body loaded correctly during the early stages of the backswing, you'll get to the top of the swing and feel like you don't have any power and because of this you'll continue to swing your arms farther in an effort to feel that your muscles are loaded, so that you'll have some power during the downswing.
The key to fixing this problem is to properly "load" the right leg and core muscles during the backswing, and it's critical to start doing this during the takeaway.
What this does is give you the feeling of muscular tension earlier in the swing and a sense of loading so that when you get to the top of the swing with your arms, you have something to cue you to start the downswing.
This sensation of load (creating tension) becomes particularly apparent in your right leg, as this triggers your brain to start moving in the other direction at the beginning of the transition.
Without this cue, you will continue to swing long and across the line, no matter how hard you try and stop.
During the takeaway, make certain that your right knee remains flexed and you feel that you're rotating on your right hip socket in such a way that you feel wound up like an elastic band.
Out of Bounds: "Never argue with an idiot. They will drag you down to their level and beat you with experience." -- Mark Twain.