Sullivan & Cromwell is advising Tokio Marine Holdings, Japan’s largest insurer by market value, on its agreement to buy U.S.-based HCC Insurance Holdings for $7.5 billion.

The acquisition would be the biggest M&A deal by a Japanese company this year, after Japan Post Holdings acquired Australian freight and logistics firm Toll Holdings for $5 billion in May.

Tokio Marine on June 10 said that it would pay $78 a share, or 1.9 times HCC’s book value as of March 31, representing a 35.8 percent premium to the U.S. insurer’s average share price over the past month.

The Japanese insurer said it would be able to greatly diversify its business portfolio through HCC, which runs a number of specialty lines of insurance, including accident and health, and directors’ and officers’ liability.

Sullivan & Cromwell is advising Tokio Marine on the deal, while Willkie Farr & Gallagher is acting for HCC.

A Debevoise & Plimpton team led by New York corporate partner William Regner is advising Evercore Partners, Tokio Marine’s financial advisor.

Tokio Marine expects to complete its acquisition between October and December this year, it said in a statement.

Houston-based HCC has 2,500 employees and had $458 million in net profit on revenue of $2.7 billion in 2014.