How to determine basis from a merger when cash and fractional shares credited to my account

p-p-kilgore,

Great question.  But the merger happened on Dec 20, 2018.  Are you trying to file or amend your 2018 return now? 

Anyhow, I always first refer to the proxy statement for the merger.  In this case you can download that statement from

https://www.cigna.com/assets/docs/annual-reports-and-proxy-statements/notice-of-special-shareholder-meeting-and-proxy-statement.pdf

Turn to page 195 and read through the section MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES

in particular page 197 covers the two cases of holders of Cigna vs holders of Express Scripts.

You should also look up the stock market pricing for the merger date.  CI on 12/20/2018 had a high of 194.81 and a low of 178.30.  It is acceptable to use their average as the fair market value of CI stock for that day.  You can also use the average of the opening and closing figures 194.81 and 179.80.

Cigna also issued a form 8937 (https://www.cigna.com/assets/docs/about-cigna/form8937.pdf) which basically regurgitates info from the proxy statement.

Hope this help get you started.