Delta Air Lines said Tuesday it has reached an agreement with Boeing Co. to cancel an order placed for 18 Boeing 787 Dreamliner aircraft that it inherited as a part of its merger with Northwest Airlines.

The 787 Dreamliner aircraft deal was estimated at $4 billion based on list prices, Atlanta-based Delta said in a statement, without disclosing the terms of the agreement or the reason behind the cancelation. The airline is continuing instead with orders already in progress for 120 narrowbody Boeing 737-900ERs through 2019.

"This business decision is consistent with Delta's fleet strategy to prudently address our widebody aircraft needs," Greg May, Delta's senior vice president of supply chain management and fleet, said in the statement.

"We've been working closely with Delta as their needs have evolved since inheriting the order from Northwest. ... Delta is a valued customer and we continue working with them to meet their future fleet requirements. Customer interest in the 787 continues to be strong, with almost 1,200 orders to date," said John Dern, a spokesman for Chicago-based Boeing, according to Bloomberg.

Although the motivation behind the decision was not officially disclosed, it is seen as part of a number of top U.S. airlines seeking to limit flight capacity growth and shrinking services in response to falling airfare. Several airlines have similarly opted out or delayed their respective deals for wider-bodied aircraft.

Earlier this year, Delta deferred acquiring four widebody Airbus Group SE A350s until 2019 and 2020, instead of the scheduled 2018. Similarly, the pick-up date for American Airlines Group's 22 A350s was also delayed by more than two years.

Aviation industry experts were not surprised by the cancelation. "I wasn't surprised, but I was surprised they took 10 years to do it," said Bob Mann, head of aviation consultant R.W. Mann & Co., according to Bloomberg.