Addison’s National Teachers Associates Life Insurance Company is being purchased by an Illinois-based public company for $405 million. Horace Mann Educators Corp. (NYSE: HMN) announced Monday its acquisition of NTA, which specializes in developing, marketing and underwriting supplemental insurance products for the education market. For the 12 months ended Sept. 30, the North Texas company had premiums of roughly $130 million and total core earnings of approximately $30 million. The acquisition,…