OS X :: Wish Programs Would Exit When Close Them?

I was wondering if there is a setting to change so when I close a program it doesn't still run on the dock.For example: If I open Numbers from the dock and then close it (by clicking the 'x' in the top left corner of the gui window), it will still be running in the background. The only way for me to close it (even after I've 'x'ed out of the window) is to right click on the Numbers icon and click on 'quit'. Since I have Numbers in the dock, visually I cannot tell that it is still running and using memory.This is fairly annoying, b/c it is easy to have many of these idle processes running in the background eating up memory and I find myself periodically having to go through my dock to see which processes are still running unbeknownst to me.