Humana Inc. said Tuesday it reached an agreement with the U.S. Department of Justice to divest certain parts of Arcadian Management Services to help preserve market competition as the agency raised concerns. Humana, a health insurer, agreed to acquire Arcadian for an undisclosed sum in August as part of its broader effort to expand in its Medicare business. The deal was expected to close in late 2011. The Justice Department said Tuesday it filed a civil suit to block the acquisition and reached the proposed settlement with the companies to divest some Arcadian businesses to allow the deal to go forward.