Yes, In fact the Idaho Batholith (formed through a series of phases, 40 to 100 mya) underlies even the Snake River Plain and beyond it to the South. But it dips down under the SRP, with the SRP on top of it: like a basin. The Idaho Balolith's 20,000 square miles underlie almost all the mountain ranges in Idaho: North, Central, and South.
The hot springs I was referring to east of Stanley are in the Salmon River: not the Snake River, but there are hot springs all through this area as well - all across Central and Southern Idea. Some of them are mineral hot springs and smell of sulfur: a clear indication of magma below. These are all along fault zones. The Salmon River follows a suture.
The earthquake that I was referring to is the 1983 Borah Peak Earthquake (7.3 on the Richter) that is just west of the Lost River Range and was centered just south of the Borah Peak (the highest peak in Idaho, elev. 12662 ft.). The Lost River and Lemni Ranges both run parallel to the Bitterroot Range and are part of the Idaho-Montana Thrust Belt: lots of fault-related folding, including the basin-range faulting from the SRP formation.
A great photo of the 1983 earthquake scarp and the Borah Peak can be seen at:
Maughan
http://www.forwolves.org/ralph/wpages/borahpk.htm
It is interesting to note that the Bitterroot and Lost River Ranges are actually part of an older orogeny system called Sevier Orogenies that were formed 150 mya when an ancient plate (now totally subducted) called the Farallon oceanic plate collided with the North American Plate. But lot's of very complex faults, folding, and errosional activity followed.
http://home.att.net/~goggallery/frames/geology2.htm
"The Bitterroots expose rocks that formed in the earth's middle crust at depths somewhere around 20-30 kilometers (12-18 miles) below the surface, 90-50 mya, when Western Montana was part of a mountain range (known as the northern Sevier Orogen) more like the Andes, much higher and more extensive than it is today. 50 mya, the Sevier Mountains were torn apart by the extension or thinning of the continental crust. As the earth's crust thinned and the mountain belt collapsed, portions of the middle crust were transported upward to the surface on very large faults that are well exposed in a flank of the Bitterroot Range."
http://clasnews.clas.ufl.edu/news/clasnotes/9910/
Another interesting fact is that the current magma chamber over Yellowstone is definitely a magma plume originating out of the Earth's outer core. This is evident by the fact that the Snake River Plain forms an arc 400 miles long and is dotted with volcanic activity, faults, and thermo-uplifts and hot springs. The Craters of the Moon National Monument is covered with 100 mile long stretches of lava and has cinder cones just 30-40 ft. high that errupted only 2,000 years ago. Compare that to the much earlier erruptions in Yellowstone. Also, basalts found on the Northwest part of the SRP are younger than those found in the Northeast part of the SRP. Therefore, the underlying plume must have formed - or still be forming - multiple magmatic chambers (hot spots) and not just the one now over Yellowstone. The 1983 Borah Earthquake is just 50 miles due North of Craters of the Moon Nat'l Mon.
"A hot spot provided the magma for the eruptions at Craters of the Moon. The Great Rift provided the pathway for the magma to reach the surface. The Great Rift is the most extensive of several volcanic rift zones which traverse the Snake River Plain. Volcanic rift zones are weak areas where the Earth's crust has stretched and thinned and fissures have developed. Magma under pressure follows these fissures to the surface. The Great Rift, which passes through Craters of the Moon, is 60 miles long and from 1-1/2 to five miles wide. It is characterized by short surface cracks, more than 25 cinder cones, and is the point of origin for over 60 lava flows."
http://www.nps.gov/crmo/hsg2.htm