Would idling my car for a while charge my battery? if so how long would you have to idle?

or do you have to drive your car to charge battery. I dont drive my car to often consequently my battery loses its charge. I would like to just idle it for maybe 20 minutes to charge battery. would 20 minutes be long enough? I guess i could take off the battery cable to battery. wpuld i have to take both positve and negative cables or can i only take off positve cable and leave on negative cable so battery does not discharge?