Heres how the Academy of Television Arts and Sciences defines a miniseries:
A miniseries is based on a single theme or story line, which is resolved within the piece. In a single awards year, all of the parts must be presented under the same title and have continuity of production supervision. A miniseries consists of two or more episodes with a total running time of at least four broadcast hours (at least 150 program minutes).
Thats pretty straightforward, and applies to both True Detective and Fargo. However, there is a caveat! The ruling on Emmy eligibility goes:
A limited-run series with a "created by" credit CANNOT enter as a miniseries unless the producer for the limited run series applies for and receives entitlement to dual qualification, i.e. qualification in more than one category, because of an affirmative determination by the Awards Committee that the limited run series has elements of both drama series and miniseries categories.
So, because True Detective executive producer Nic Pizzolatto has a created by credit a classification the Writers Guild of America gives to the creators of a regular series True Detective is eligible only for the Emmys drama series category by default. In fact, Pizzolatto would have had to petition ATASs awards committee to move out of the drama race and into the miniseries category. Thats what American Horror Story co-creator Ryan Murphy had to do for American Horror Story, which was revealed to be a closed anthology series after the first seasons story concluded. As for Fargo, the credit executive producer Noah Hawley received reads created for television by and because thats not the exact same thing as created by, the show was free to submit into the miniseries category without fuss.