Idaho has tremendous fly-fishing—some of the best in the world. And it covers all sorts of water, from fast, high-mountain streams to lowland desert reservoirs. That also means tough terrain, and you can expect to work up a sweat before you let that fly loose. I’d recommend waders that are light and offer some measure of breathability.
Patagonia’s Watermaster Waders ($299) are a great example. They’re Patagonia’s lightest wader, combining the company's excellent H2No Hydrostorm barrier with a puncture-resistant polyester microfiber. They come with heavier material lower down, where constant soaking is more likely, and lighter material higher up, providing breathability. They also have an easily adjustable fit and gravel guards.