2012 Subaru Forester 2.5x (4-cylinder, naturally aspirated, 4-speed automatic)

I recently bough a used Subaru Forester with around 58,000 miles on it. After a couple weeks of driving it, I've noticed that, when I start it up cold in the morning or after work, it idles rough at around 1500 RPM. It will stay that way for a good long while if I just let it idle, but when I shift out of park and then back into park, the idle smooths out and drops down to around 500 RPM, which is where it sits when it's warmed up. Other than that everything seems to work smoothly on it. Air filter looked pretty clean when I checked it, but I replaced it anyway. Still idles rough though.