Found this searching Southwest Heavy Maint. It was an article written in Sept 2012, regarding Southwest taking control of Airtran.
In addition to a common fleet of 737-700s, the maintenance infrastructures of the merging airlines offered similar capabilities, according to Brian Hirshman, Southwest's senior vice president of technical operations. AirTran operated line-maintenance hangars at Atlanta and Orlando, but it outsourced all airframe and component work. Southwest performs heavy airframe work and intermediate level inspections (C checks) at Dallas Love Field. Intermediate checks are also carried out at Houston's William P. Hobby Airport, Chicago's Midway Airport, and Sky Harbor Airport in Phoenix—but Southwest contracts out “a large majority” of component repairs. (Emergency slides, crew seats, aircraft doors and on-wing composite repairs are among components maintained in-house. Both carriers outsource engine maintenance. Southwest expects to add line maintenance at Denver International Airport in October.[/COLOR]