Here’s what I found in the OED entry for “idyll”:

etymology: Gr. {epsilon}{ilenis}{delta}{guacu}{lambda}{lambda}{iota}{omicron}{nu} a short descriptive poem, dim. of {epsilon}{ilenisfrown}{delta}{omicron}{fsigma} form, picture.

1. A short poem, descriptive of some picturesque scene or incident, chiefly in rustic life. prose idyll, a prose composition treating subjects of the same kind in a poetic style. (and derivs.).

2. transf. An episode or a series of events or circumstances of pastoral or rural simplicity, and suitable for an idyll.

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.