This livery is meant to pay tribute to the State of Illinois. Southwest apparently has quite the presence at Chicago - Midway.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/44389375@N00">Shawn</a> from Airdrie, Canada
CC BY-SA 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/cdnav8r/3398411031/">Southwest's "Illinois One" @ LAX</a>