Nippon Life Insurance plans to acquire all the shares it does not own in Resolution Life Group Holdings for about $8.2 billion — an all-cash deal that gives Japan's biggest insurer a firmer footing in the U.S. market.

The deal is the largest overseas acquisition by a Japanese insurer to date and the second major transaction announced by Nippon Life this year. Japanese insurers are keen to expand abroad given limited growth prospects at home due to a shrinking and aging population.

"For some time we have wanted to own a business that could be core in the largest global insurance market — the U.S.A," Hiroshi Shimizu, Nippon Life's president, told a news conference on Wednesday.