Kari, thanks for raising this interesting question, about which I’ve been ruminating. And yes, I am thinking about “ruminate” in both the sense of “thinking” and the sense of “chewing cud.” As Etymology Fan mentioned, early in its history, the English word “idyll,” like its Greek root, referred to life in the countryside, and specifically to poems written about that kind of life. So it connotes a bit of the pastoral (orignally having to do with shepherds, like the “pastor” who shepherds a church) and the bucolic (originally having to do with cowherds, a relative of “bovine”).
I suspect you’re right that in many people’s minds, “idyllic” is probably reinforced by “ideal,” and probably regarded as pretty much synonymous. But I think the history of the word gives it a more specific flavor, and it’s worth distinguishing it. I’d tend to use “idyllic” when I want to add a certain amount of nostalgia and wistfulness to something that also happens to be ideal. Does that make sense? (I also think “idyllic” is often overused to the point of near-meaninglessness, particularly by travel writers. What do the rest of you think?)
In any case, Kari, I appreciate your raising this question. I hadn’t really thought through the distinction before.