Hello my 96 Lexus ES 300 is idling under 700.

Hello my 96 Lexus es 300 is idling under 700 rmps when I first start it up in the morning but when it warms up it idles were it's suppose to idle. My car use to idle at 1200 rmps and once it starts to warm up it drops to the right rmps, what would cause it to idle at a lower rmp in the morning when it wasn't doing this at first and it just started doing this.