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E. idahoensis is a member of the detersa group in the subgenus Euxoa. They are rather robust, medium-size moths (3.2-3.5 cm wingspan) with charcoal brown or sooty brown forewings. The costa is light grey or grey brown and contrasts with the ground, especially over the basal half. There is a prominent black basal dash, and the area in the discal cell before the orbicular and between the orbicular and the reniform spot is black. The orbicular spot is prominent, usually open to and concolorous with the pale costa. The crescent shaped reniform is less contrasting and is finely outlined with black scales in most specimens. The normal lines are absent, except for traces of the subterminal line indicated by a few dark saggitate spots. The hindwings are sooty brown with a darker discal mark. They are very similar to E. castanea, with the chestnut of E. castanea replaced by charcoal brown in idahoenesis. Until recently (2000) "idahoensis" was comprised of what is now recognized as a group of at least 6 species, most of which are being described as new. True idahoensis is larger, darker, less streaked, has a larger orbicular, is more charcoal brown in color and has broader ovipositor lobes than the other 5 species. Because of this taxonomic confusion, previously published information regarding idahoensis may refer to any or all of the 6 species in the complex, including idahoensis. Bowman (1951) listed idahoensis as a subspecies of E. costata.

The extent of the range of idahoensis cannot be determined from the published literature, as what has been treated in the literature as idahoensis is now treated as a complex of at least 6 species, most of which have not yet been described. E. idahoensis occurs in Alberta in the foothills and mountains, as well as in wooded parts of the grasslands region. It is not known to occur east of Alberta.