Antero Resources (NYSE:AR) is up 4.9% after hours alongside word that the energy stock is joining the S&P MidCap 400 index.
It's replacing Alleghany (NYSE:Y), which is set to be acquired by S&P 500 constituent Berkshire Hathaway. That deal should be completed soon, pending final closing conditions.
Antero's move into the MidCap 400 is effective prior to the open of trading on Thursday, Oct. 20.