Health insurer Aetna Inc, which is in the process of buying smaller rival Humana Inc, said on Monday its Obamacare insurance business improved at the end of last year, providing a stronger starting point for 2016. Aetna said its individual insurance sold on the exchanges created by the Affordable Care Act, also called Obamacare, had operating losses of about 3 to 4 percent in 2015 as fourth-quarter operations improved from mid-single digit losses earlier in the year.