The answer is: Possible, but contrive
Let's face it, detrivore is not a good option for the kind of energy needed to support a large brain / body ratio. By the time they eat, their food have already pass through at least 2, probably more trophic level (plant => herbivore => carnivore (continue for more carnivores)). Combine that with the fact that what the detritivore eat is waste (ie. not as good as the organism itself, and laden with substances that is at least harmful to the excreting organism), there is not much left for them to pick over here. On the other hand, a big, well-developed nervous system is an important foundation for intelligence.
Enter the slime mold, eater of various organic scrap, and surprisingly intelligent (not human level though). They can solve mazes, choose healthy foods, and plan ahead of time (so at the very least, they are smarter than me :o). Currently still under study, so we don't quite know how they do those things yet, but I think being a collection of multi-nuclei cell with capability for cooperation as a colony play a big part in it.
Another thing you might want to know about intelligence is why the organism develop it in the first place. Plant can gain lots of energy from the sun, which means they only need to be smart enough to search for light, water, and nutrients and out-compete other plant for it, so they don't need much intelligence. On the other end of the spectrum, carnivore need to compete with both their prey, and other carnivores of the same trophic level or above it, so they tend to be more clever. The only main challenge for detrivore are environmental conditions and predators, so not a lot to go on here (even the slime molds above only become multicellular if the condition are harsh, which affect everyone). The good news is intelligence reward having more intelligence, so as long as you make the first step, things are looking up from there.
So sure, if the planet you are imagining is ridiculously abundant, but only in a few specific spots, and harsh in everywhere else, with energetic sun to support a lot of energy going through different energy level, but damaging in places without plantlife (not impossible), and somehow the carnivores don't develop intelligence first (that will be a problem), then yeah, you can have an intelligent detrivore that's as smart as human (their expression of it will be different though), but bigger in size, and probably composed of many less intelligent smaller organism that might bail if the condition become too plentiful though.
There is a simple, hand-wavey solution to those problem by just making the detritivore organism have extremely different metabolism than the rest of the biosphere, so that a normal, plentiful, temperate environment for everyone else is a harsh place with bountiful spots to them, and somehow their vital nutrients are indigestible to everything else. But then you have to explain why they are the odd one out in the entire biosphere.
Additional information: There is an inherent problem with the intelligent detrivore though, depend on their tech-level in your story. If they have advance enough to overcome other flora and fauna of you world, then there is a good chance that they won't stay detrivore, but develop agriculture and animal domestication, and eventually become omnivore (like us) instead. Therefore take their intended culture into account before you make the species.