Private equity firm Blackstone Group L.P. agreed to buy privately held insurance brokerage Alliant Insurance Services, Inc., it said late Sunday.

Blackstone is buying Alliant, from private equity firm Lindsay Goldberg and Alliant's management and employees. While terms were not disclosed, a source close to the process said Blackstone will pay $1.2 Billion, according to Reuters.

Alliant is an attractive platform due to its market leadership in its core segments (and) superior growth prospects, Blackstone's Senior Managing Director Chinh Chu said in a statement.

Alliant is one of the largest US insurance brokerage firms. It distributes employee benefit products to more than 20,000 commercial clients world wide.

The transaction is expected to close within the next 75 days.