Shares of Aetna (AET) tumbled to finish Monday's regular trading session in the red after the company acquired Humana (HUM), making it TheStreet's Move of the Day. Aetna, one of the biggest health insurance companies in the U.S. is buying the country's second largest provider of private Medicare insurance. Aetna announced it will buy its insurance rival Humana for $37 billion, an equivalent of about $230 per share. The acquisition makes the combined company the largest Medicare player in the industry. Aetna's shareholders will hold roughly 74 percent of the combined company, while Humana's shareholders will own the remaining 26 percent. The merger comes as other health insurers consider consolidating to reduce costs. TheStreet's Kurumi Fukushima reports in New York.