Question:  As of September 2017, the Delta Air Lines mainline fleet comprises 852 aircraft in service, merging with Northwest Airlines in 2009, Delta Air Lines operated an all-Boeing and McDonnell Douglas fleet; it incorporated into which company's Airbus aircraft into its fleet following the merger?
Sentence (from "Delta Air Lines fleet"): Delta is known for its policy of generally buying older generation or used aircraft and for continuing to fly aircraft for 20–30 years, much longer than most other major airlines.
Relevant evidence: No