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A medium-size (wingspan approx. 3 cm) broad-winged dull light and darker yellow-brown moth with a glossy sheen. The forewings are pale yellow-brown, overlaid with dull sooty brown on the basal third and in a broad band across the post-media area. The outer edge of the postmedian band is well-defined and erratic, followed by a thin broken terminal line. The hindwings are similarly marked, but less well defined. The wing fringes are light brown. The entire pattern is poorly defined and "cloudy". Idia lubricalis (Gey.) (see below) is a smaller, darker moth. Some specimens of occidentalis have more dark scaling and a more complete pattern than is evident on the illustrated specimen. Idia immaculalis is pale tan without any pattern. Two other species with a similar color and sheen include Protoperigea posticata (Harv.) and Pronoctua peabodyae.

Very poorly known. There is a single brood in Alberta, and adults come to light. The larval hosts are unrecorded, but related species feed on fungi and mold on damp and decaying vegetable matter. Alberta specimens have been collected in mid August.

Although the eastern Idia lubricalis is usually much smaller and darker than Alberta occidentalis, some specimens may be difficult to separate. Idia lubricalis occurs mainly in the aspen parklands region of Alberta, but also in wooded areas in the grasslands region. Both species have been collected at Dinosaur Provincial Park, with lubricalis in wooded riparian areas and occidentalis in dry open areas. Until recently I. occidentalis was treated as a subspecies of I. lubricalis, but both genitalic characters and DNA barcoding show them to be good species.