Simpson Thacher & Bartlett is representing Japan's Sumitomo Life Insurance Co. on a $3.7 billion acquisition of U.S. insurer Symetra Financial Corp.

Osaka-based Sumitomo will pay $32 apiece for all the outstanding shares of Symetra, a 32 percent premium to the New York Stock Exchange-listed company's average price for the past month. Based in Bellevue, Washington, Symetra manages retirement plans, employee benefits, annuities and life insurance products to 6.8 million customers in the United States.

The deal is the latest among leading Japanese life insurers making acquisitions in the U.S. Japan's shrinking domestic life insurance market, due to an aging population, has prompted the top players to look overseas for growth. In a similar deal announced last month, Tokyo-based Meiji Yasuda Life Insurance Co., one of Sumitomo's closest rivals, agreed to buy Portland, Oregon-based StanCorp Financial Group Inc. for $5 billion. Last year, Dai-ichi Life Insurance Co. paid $5.7 billion for Birmingham, Alabama-based insurer Protective Life Corp.