Found the answer to my question from the other day. Looking at pages 19 and 20 of the 2Q18 supplemental, much of the 18.7% of GLA that is not leased is not under development. Here are the properties with termination dates before August 2018 that are not currently being developed:
Square Feet Property
118,200 Alpena, MI
118,800 Chicago, IL (S Kedzie)
96,600 Deming, NM
87,800 Manistee, MI
94,800 Riverton, WY
92,700 Sault Sainte Marie, MI
187,179 Chapel Hill, OH
137,499 Concord, NC
79,102 Detroit Lakes, MN
94,885 Elkins, WV
96,066 Kenton, OH
112,505 Kissimmee, FL
90,010 Layton, UT
76,853 Leavenworth, KS
87,500 Muskogee, OK
68,334 Owensboro, KY
94,841 Platteville, WI
94,500 Riverside, CA (Iowa Ave.)
72,511 Sioux Falls, SD
161,700 Burnsville, MN
293,700 Chicago, IL (N Harlem)
155,100 Johnson City, NY
194,900 Lafayette, LA
208,700 Mentor, OH
351,600 Middleburg Heights, OH
215,000 Overland Park, KS
204,500 Sarasota, FL
209,900 Toledo, OH
82,000 York, PA
166,000 Friendswood, TX
215,000 Westwood, TX
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4,358,785
4,358,785/36,390,000 is 12%.