please see the link from Cigna on how to treat the change in cost basis on the stock and the cash. it specifically states that 2018 is the year of the transaction, so the answer to your question is 'no' and I suspect you can't document that it was received in 2019 (the check is dated 2018 and the envelope is probably dated 2018)
https://www.cigna.com/assets/docs/about-cigna/form8937.pdf
further it could get trickly on how to re-set the cost basis and what the gain is for this year's tax is - please read the link below closely
https://community.quicken.com/discussion/7834047/merger-of-cigna-and-express-scripts