This may sound like kind of a silly question, but are snow pants designed to be worn over regular pants, or by themselves?

I only ask because I have a pair of thermal underwear to put on under my normal pants, but I'd like to get a shell of some sort to use over top of them (in conjunction with snow boots) to help serve as a water barrier when tromping around in shin high snow (I had the pleasure of experiencing winter in Minnesota instead of the more familiar Tennessee!)