Sullivan & Cromwell is advising Japanese insurer Tokio Marine Holdings Inc. on its proposed $7.5 billion acquisition of Houston-based HCC Insurance Holdings Inc.

Tokio Marine is offering $78 per share for HCC, which is a 37.6 percent premium to the target company's $55.69 closing price on June 9. The transaction, subject to shareholder and regulatory approval, is expected to close in the fourth quarter.

In a statement, Tokio Marine said the acquisition allows the company to further diversify its geographic footprint and gain access to specialty insurance markets such as accident and health, directors' and officers' liability, agriculture and aviation, among others.