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Even though common sense would tell you as much, I had never heard of detraction being a sin, until I began to learn about the Catholic Faith.

This was one thing that attracted me to Catholicism.  As I just told my mother a little while ago (I am doing home care for my father this morning), it is not so much that Catholicism differs from other Christian [sic] sects on the matter of "being a good person" or not, it is more that our morality is systematized, logic and reason are employed, and most of all, it is utterly dispassionate --- the same "rules" for everyone, there is none of this "this one believes this" and "that one believes that", it's black-and-white where it's black-and-white, and it's gray where it's gray.  I have a pet theory that this is why educated Catholics make so much better lawyers and judges (six, seven if you count Gorsuch, Supreme Court Justices are Catholic) --- our faith, and our way of thinking, is utterly objective, and we make fair decisions instead of tendentious ones.  (Sotomayor is a product of her times, upbringing, and culture.)

Degrees of gravity in both calumny and detraction:

DETRACTION
Just know that detraction can be either mortal or venial, depending on the severity of the behavior being reported, combined with the role/status of the person being detracted against, which may compound the detraction.

Thus, revealing a fault that you alone witnessed in so-and-so lay person and then passed along to another lay person (an equal of both of you) would normally be confessed as a venial sin.  However, if there was no cause or provocation in which to reveal that fault, one should examine one's conscience about the motive involved: Pride? (competition with that other) A private wish to stir up enmity with tale-bearing (another sin)? Envy? (an opportunity to diminish the stature of the other)

Heavier level: revealing a grave fault or grave sin in another lay person: Mortal sin.

Compounded gravity: revealing a hitherto unknown grave fault or grave sin in a consecrated person: Mortal sin.  By definition, if the revelation involves just protection of a vulnerable lay person, then the revelation is not detraction but an act of charity and justice.  However, merely revealing a grave problem or moral condition of a priest or other religious, for the purpose of spite, attention, or vanity would be mortally sinful.

CALUMNY
Given that calumny is by definition a lie, it seems not possible for it to be light matter because we are potentially destroying the reputation of another through falsehood.  It's a combination of two violations of justice and at least one violation of charity.

Compounded level: involving a consecrated person as either the object of false information or the receiver of false information about a member of that person's flock or similar subordinate relationship.

Three times in the last 8 years I have been the object of calumny at my parish, and in all three occasions that did include deliberate involvement of priests as the receivers of falsehoods.  I am not the only one to have suffered calumny at this parish; at least one other woman has been the object, more often than I, and over the same time period. The calumny was motivated by Envy in all of these cases.  The point is, the reason that the calumny is compounded in that case is that it corrupts the relationship of a lay person with his or her priest & often spiritual director and confessor. 

Edited to add that the previous sentence comes from a sermon about this by Fr. Ripperger.