ERJ-175 pilot reported an uncommanded disconnect from the tug during the push back maneuver resulting in the aircraft rolling back around 20 feet; before brakes were applied.

Narrative

During Pushback; shortly after we began moving straight back; the tug became disconnected from the towbar and the aircraft rolled back straight approx 20ft until I could smoothly apply the toe brakes to bring it to a stop. We also lost communication (loud squeal) as the headset cord became disconnected during the event. As soon as I could see the tractor in front of us with no towbar visible; I knew what happened and carefully applied the brakes to come to a gentle stop and signaled the ramp crew that the brakes were set. I opened the window and asked what happened and was told 'they forgot to put the pin in the towbar.' They asked if we could power out from our current position; which was a little shorter than the usual pushback position; but acceptable. I agreed; and once they disconnected the towbar from the airplane and we got the all clear; we started up and began taxiing. Note: this was NOT the towbar shear pin; this was the pin that attaches the towbar to the tug. The towbar shear pin was not broken; so the broken shear pin procedures were not applicable. The pushback was very smooth and would have felt like a normal pushback to a passenger sitting in the back. It's just that during the last few feet of the push we were rolling unattached to the tractor. This event is a good example of why we need to be heads up during pushback. I can see a situation where if a flight crew was not aware of the disconnect; the airplane might have rolled back into another aircraft or even off the paved surface. Strangely; I didn't notice any signals from the wingwalkers when we became disconnected. I would have expected them to be jumping up and down to get my attention and signaling for me to stop the aircraft. I don't believe we have a procedure for a 'breakaway' event during push; but it might be a good thing to include in the FOM. The company has a breakaway procedure in their FOM.