Tokio Marine Holdings Inc. agreed to buy HCC Insurance Holdings Inc. for about $7.5 billion (about ¥941.3 billion) in the biggest acquisition by a Japanese insurer, stepping up an overseas expansion to counter stagnation at home.

Tokio Marine will pay $78 in cash per share for Houston, Texas-based HCC, according to a statement on Wednesday. That's a 37.6 percent premium to the latest closing price.

Japanese insurers are looking for ways to spread risks and increase revenues overseas as an aging population saps demand for policies and natural disasters lead to higher payouts. To that end, they've announced $27.5 billion of acquisitions abroad in the past five years, Bloomberg-compiled data show.