The U.S. Justice Department announced Monday it reached a settlement with British energy company BP over the 2010 Deepwater Horizon oil spill.

U.S. Attorney General Loretta Lynch in July said both parties reached an agreement in principle to settle civil claims for the spill. If agreed to, the estimated $18.7 billion would be the largest settlement of its kind in U.S. history.

The U.S. Supreme Court, in one of the final decisions of the previous term, refused to hear two separate cases filed by BP and Anadarko Petroleum challenging fines related to the spill.

Today, 11am: AG @LorettaLynch, fed partners to announce BP settlement, #GulfRestoration plan http://t.co/gJS7W44UgC— Justice Department (@TheJusticeDept) October 5, 2015

The nation's highest court let stand a decision from a New Orleans court, which in June 2014 said the companies are liable for penalties under the Clean Water Act.

The cases filed against the federal government by BP and Anadarko were Nos. 14-1217 and 14-1167, respectively. Anadarko held a minority stake in the Macondo well beneath the Deepwater Horizon rig in the Gulf of Mexico.

The U.S. District Court of the Eastern District of Louisiana ruled last year that BP released 3.2 million barrels of oil into the Gulf of Mexico in 2010, about 1 million barrels less than the government estimated.

BP said in an early 2015 statement it suspected the penalty would be "at the lower end of the statutory range."

The district court ruled in September that BP's activities at the Macondo well beneath the Deepwater Horizon rig in the Gulf of Mexico amounted to willful misconduct

The spill left 11 rig workers dead. A cascading series of failures at the Macondo well resulted in the worst incident of its kind in the history of the industry.