Farmington specialty insurance group Darwin Professional Underwriters has been acquired by Bermuda-based Allied World Assurance Company Holdings, which has an office in Boston, in a deal expected to nearly double Allied's U.S. presence, the companies said.

Darwin's business model for underwriting small professional liability businesses is also expected to complement Allied's large account, specialty insurance and reinsurance businesses.

Under the terms of the merger agreement, security holders of Darwin will receive approximately $550 million in cash, or $32.00 per share. The transaction is expected to be completed sometime during the fourth quarter of this year.

Darwin's President and Chief Executive Officer Stephen J. Sills will retire after the transaction is closed. Jack Sennott, Darwin's chief financial officer, will become the chief operating officer when the deal closes.