Go to the General Electric website & find the publication entitled "The Art and Science of Protective Relaying". When you are reading this, bear in mind that IDMT is from British/IEC practice - the acronym stands for "Inverse Definite Minimum Time" and translates roughly to a standard Inverse relay curve in ANSI terms.
If you want more details, use your search engine to find other relay manufacturers - the truth IS out there.