SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Real estate and accumulated depreciation
Encumbrances	$ 136,643
Initial Cost to Company
Land	252,146
Buildings and Equipment	1,320,057
Costs Capitalized Subsequent to Acquisition	123,931
Cost Amount Carried at Close of Period
Land	253,058
Buildings and Equipment	1,443,074
Total	1,696,132	$ 1,682,480	$ 1,568,562	$ 1,467,863
Accumulated Depreciation	(255,879)	$ (219,791)	$ (187,635)	$ (156,661)
Aggregate cost for federal income tax purposes	$ 1,906,441
Useful life of buildings and improvements	40 years
Useful life of equipment	12 years
131 Clayton Street, Montgomery, AL
Initial Cost to Company
Land	$ 920
Buildings and Equipment	9,084
Costs Capitalized Subsequent to Acquisition	16
Cost Amount Carried at Close of Period
Land	920
Buildings and Equipment	9,100
Total	10,020
Accumulated Depreciation	(1,024)
4344 Carmichael Road, Montgomery, AL
Initial Cost to Company
Land	1,374
Buildings and Equipment	11,658
Cost Amount Carried at Close of Period
Land	1,374
Buildings and Equipment	11,658
Total	13,032
Accumulated Depreciation	(583)
15451 North 28th Avenue, Phoenix, AZ
Initial Cost to Company
Land	1,917
Buildings and Equipment	7,416
Costs Capitalized Subsequent to Acquisition	28
Cost Amount Carried at Close of Period
Land	1,917
Buildings and Equipment	7,444
Total	9,361
Accumulated Depreciation	(248)
711 14th Avenue, Safford, AZ
Initial Cost to Company
Land	460
Buildings and Equipment	11,708
Costs Capitalized Subsequent to Acquisition	189
Cost Amount Carried at Close of Period
Land	460
Buildings and Equipment	11,897
Total	12,357
Accumulated Depreciation	(1,619)
5045 East Butler Street, Fresno, CA
Initial Cost to Company
Land	7,276
Buildings and Equipment	61,118
Costs Capitalized Subsequent to Acquisition	58
Cost Amount Carried at Close of Period
Land	7,277
Buildings and Equipment	61,175
Total	68,452
Accumulated Depreciation	(20,438)
10949 N. Mather Boulevard, Rancho Cordova, CA
Initial Cost to Company
Land	562
Buildings and Equipment	16,923
Cost Amount Carried at Close of Period
Land	562
Buildings and Equipment	16,923
Total	17,485
Accumulated Depreciation	(917)
9800 Goethe Road, Sacramento, CA
Initial Cost to Company
Land	1,550
Buildings and Equipment	12,263
Costs Capitalized Subsequent to Acquisition	1,491
Cost Amount Carried at Close of Period
Land	1,550
Buildings and Equipment	13,754
Total	15,304
Accumulated Depreciation	(2,285)
9815 Goethe Road, Sacramento, CA
Initial Cost to Company
Land	1,450
Buildings and Equipment	9,465
Costs Capitalized Subsequent to Acquisition	1,523
Cost Amount Carried at Close of Period
Land	1,450
Buildings and Equipment	10,988
Total	12,438
Accumulated Depreciation	(1,157)
Capital Place, Sacramento, CA
Initial Cost to Company
Land	2,290
Buildings and Equipment	35,891
Costs Capitalized Subsequent to Acquisition	4,163
Cost Amount Carried at Close of Period
Land	2,290
Buildings and Equipment	40,054
Total	42,344
Accumulated Depreciation	(5,939)
4181 Ruffin Road, San Diego, CA
Initial Cost to Company
Land	5,250
Buildings and Equipment	10,549
Costs Capitalized Subsequent to Acquisition	3,736
Cost Amount Carried at Close of Period
Land	5,250
Buildings and Equipment	14,285
Total	19,535
Accumulated Depreciation	(2,180)
4560 Viewridge Road, San Diego, CA
Initial Cost to Company
Land	4,269
Buildings and Equipment	18,316
Costs Capitalized Subsequent to Acquisition	957
Cost Amount Carried at Close of Period
Land	4,347
Buildings and Equipment	19,195
Total	23,542
Accumulated Depreciation	(9,159)
Sky Park Centre, San Diego, CA
Initial Cost to Company
Land	685
Buildings and Equipment	5,530
Costs Capitalized Subsequent to Acquisition	1,526
Cost Amount Carried at Close of Period
Land	685
Buildings and Equipment	7,056
Total	7,741
Accumulated Depreciation	(1,872)
Turning Basin Business Park, Stockton, CA
Initial Cost to Company
Land	563
Buildings and Equipment	5,470
Cost Amount Carried at Close of Period
Land	563
Buildings and Equipment	5,470
Total	6,033
Accumulated Depreciation	(467)
16194 West 45th Street, Golden, CO
Initial Cost to Company
Land	494
Buildings and Equipment	152
Costs Capitalized Subsequent to Acquisition	6,457
Cost Amount Carried at Close of Period
Land	495
Buildings and Equipment	6,608
Total	7,103
Accumulated Depreciation	(2,926)
12795 West Alameda Parkway, Lakewood, CO
Real estate and accumulated depreciation
Encumbrances	6,344
Initial Cost to Company
Land	2,640
Buildings and Equipment	23,777
Costs Capitalized Subsequent to Acquisition	1,045
Cost Amount Carried at Close of Period
Land	2,640
Buildings and Equipment	24,822
Total	27,462
Accumulated Depreciation	(3,675)
Corporate Center, Lakewood, CO
Initial Cost to Company
Land	2,886
Buildings and Equipment	27,537
Costs Capitalized Subsequent to Acquisition	3,849
Cost Amount Carried at Close of Period
Land	2,887
Buildings and Equipment	31,385
Total	34,272
Accumulated Depreciation	(9,754)
20 Massachusetts Avenue, Washington, DC
Initial Cost to Company
Land	12,008
Buildings and Equipment	51,528
Costs Capitalized Subsequent to Acquisition	20,956
Cost Amount Carried at Close of Period
Land	12,228
Buildings and Equipment	72,265
Total	84,493
Accumulated Depreciation	(29,531)
625 Indiana Avenue, Washington DC, DC
Initial Cost to Company
Land	26,000
Buildings and Equipment	25,955
Costs Capitalized Subsequent to Acquisition	4,856
Cost Amount Carried at Close of Period
Land	26,000
Buildings and Equipment	30,808
Total	56,808
Accumulated Depreciation	(4,149)
7850 Southwest 6th Court, Plantation, FL
Initial Cost to Company
Land	4,800
Buildings and Equipment	30,592
Costs Capitalized Subsequent to Acquisition	383
Cost Amount Carried at Close of Period
Land	4,800
Buildings and Equipment	30,975
Total	35,775
Accumulated Depreciation	(3,619)
8900 Grand Oak Circle, Tampa, FL
Real estate and accumulated depreciation
Encumbrances	9,199
Initial Cost to Company
Land	1,100
Buildings and Equipment	11,773
Costs Capitalized Subsequent to Acquisition	169
Cost Amount Carried at Close of Period
Land	1,100
Buildings and Equipment	11,942
Total	13,042
Accumulated Depreciation	(1,562)
181 Spring Street NW, Atlanta, GA
Initial Cost to Company
Land	4,047
Buildings and Equipment	20,017
Costs Capitalized Subsequent to Acquisition	1
Cost Amount Carried at Close of Period
Land	4,048
Buildings and Equipment	20,017
Total	24,065
Accumulated Depreciation	(1,710)
Corporate Square, Atlanta, GA
Initial Cost to Company
Land	3,996
Buildings and Equipment	29,762
Costs Capitalized Subsequent to Acquisition	4,921
Cost Amount Carried at Close of Period
Land	3,996
Buildings and Equipment	34,683
Total	38,679
Accumulated Depreciation	(9,274)
Executive Park, Atlanta, GA
Initial Cost to Company
Land	1,521
Buildings and Equipment	11,826
Costs Capitalized Subsequent to Acquisition	3,872
Cost Amount Carried at Close of Period
Land	1,521
Buildings and Equipment	15,698
Total	17,219
Accumulated Depreciation	(3,829)
One Georgia Center, Atlanta, GA
Initial Cost to Company
Land	10,250
Buildings and Equipment	27,933
Costs Capitalized Subsequent to Acquisition	231
Cost Amount Carried at Close of Period
Land	10,250
Buildings and Equipment	28,164
Total	38,414
Accumulated Depreciation	(3,021)
4712 Southpark Boulevard, Ellenwood, GA
Initial Cost to Company
Land	1,390
Buildings and Equipment	19,635
Costs Capitalized Subsequent to Acquisition	7
Cost Amount Carried at Close of Period
Land	1,390
Buildings and Equipment	19,642
Total	21,032
Accumulated Depreciation	(1,677)
South Vinnell Way, Boise, ID
Initial Cost to Company
Land	3,390
Buildings and Equipment	29,026
Costs Capitalized Subsequent to Acquisition	474
Cost Amount Carried at Close of Period
Land	3,391
Buildings and Equipment	29,499
Total	32,890
Accumulated Depreciation	(2,470)
2020 S. Arlington Heights, Arlington Heights, IL
Initial Cost to Company
Land	1,450
Buildings and Equipment	13,160
Costs Capitalized Subsequent to Acquisition	914
Cost Amount Carried at Close of Period
Land	1,450
Buildings and Equipment	14,074
Total	15,524
Accumulated Depreciation	(2,177)
Intech Park, Indianapolis, IN
Initial Cost to Company
Land	4,170
Buildings and Equipment	68,888
Costs Capitalized Subsequent to Acquisition	2,615
Cost Amount Carried at Close of Period
Land	4,170
Buildings and Equipment	71,503
Total	75,673
Accumulated Depreciation	(7,662)
400 State Street, Kansas City, KS
Initial Cost to Company
Land	640
Buildings and Equipment	9,932
Costs Capitalized Subsequent to Acquisition	3,033
Cost Amount Carried at Close of Period
Land	640
Buildings and Equipment	12,965
Total	13,605
Accumulated Depreciation	(1,689)
7125 Industrial Road, Florence, KY
Initial Cost to Company
Land	1,698
Buildings and Equipment	11,722
Costs Capitalized Subsequent to Acquisition	10
Cost Amount Carried at Close of Period
Land	1,698
Buildings and Equipment	11,732
Total	13,430
Accumulated Depreciation	(880)
251 Causeway Street, Boston, MA
Initial Cost to Company
Land	5,100
Buildings and Equipment	17,293
Costs Capitalized Subsequent to Acquisition	984
Cost Amount Carried at Close of Period
Land	5,100
Buildings and Equipment	18,277
Total	23,377
Accumulated Depreciation	(2,377)
75 Pleasant Street, Malden, MA
Initial Cost to Company
Land	1,050
Buildings and Equipment	31,086
Costs Capitalized Subsequent to Acquisition	118
Cost Amount Carried at Close of Period
Land	1,050
Buildings and Equipment	31,204
Total	32,254
Accumulated Depreciation	(4,365)
25 Newport Avenue, Quincy, MA
Initial Cost to Company
Land	2,700
Buildings and Equipment	9,199
Costs Capitalized Subsequent to Acquisition	573
Cost Amount Carried at Close of Period
Land	2,700
Buildings and Equipment	9,772
Total	12,472
Accumulated Depreciation	(1,174)
One Montvale Avenue, Stoneham, MA
Initial Cost to Company
Land	1,670
Buildings and Equipment	11,035
Costs Capitalized Subsequent to Acquisition	1,209
Cost Amount Carried at Close of Period
Land	1,670
Buildings and Equipment	12,244
Total	13,914
Accumulated Depreciation	(1,592)
4201 Patterson Avenue, Baltimore, MD
Initial Cost to Company
Land	900
Buildings and Equipment	8,097
Costs Capitalized Subsequent to Acquisition	3,339
Cost Amount Carried at Close of Period
Land	901
Buildings and Equipment	11,435
Total	12,336
Accumulated Depreciation	(3,703)
20400 Century Boulevard, Germantown, MD
Initial Cost to Company
Land	2,305
Buildings and Equipment	9,890
Costs Capitalized Subsequent to Acquisition	499
Cost Amount Carried at Close of Period
Land	2,347
Buildings and Equipment	10,347
Total	12,694
Accumulated Depreciation	(4,765)
3300 75th Avenue, Landover, MD
Real estate and accumulated depreciation
Encumbrances	23,499
Initial Cost to Company
Land	4,110
Buildings and Equipment	36,371
Costs Capitalized Subsequent to Acquisition	483
Cost Amount Carried at Close of Period
Land	4,110
Buildings and Equipment	36,854
Total	40,964
Accumulated Depreciation	(5,364)
1401 Rockville Pike, Rockville, MD
Initial Cost to Company
Land	3,251
Buildings and Equipment	29,258
Costs Capitalized Subsequent to Acquisition	7,318
Cost Amount Carried at Close of Period
Land	3,248
Buildings and Equipment	36,579
Total	39,827
Accumulated Depreciation	(14,092)
2115 East Jefferson Street, North Bethesda, MD
Initial Cost to Company
Land	3,349
Buildings and Equipment	11,152
Costs Capitalized Subsequent to Acquisition	206
Cost Amount Carried at Close of Period
Land	3,349
Buildings and Equipment	11,358
Total	14,707
Accumulated Depreciation	(651)
Rutherford Business Park, Windsor Mill, MD
Initial Cost to Company
Land	1,598
Buildings and Equipment	10,219
Costs Capitalized Subsequent to Acquisition	8
Cost Amount Carried at Close of Period
Land	1,598
Buildings and Equipment	10,227
Total	11,825
Accumulated Depreciation	(789)
Meadows Business Park, Woodlawn, MD
Initial Cost to Company
Land	3,735
Buildings and Equipment	21,509
Costs Capitalized Subsequent to Acquisition	261
Cost Amount Carried at Close of Period
Land	3,735
Buildings and Equipment	21,770
Total	25,505
Accumulated Depreciation	(2,662)
11411 E. Jefferson Avenue, Detroit, MI
Initial Cost to Company
Land	630
Buildings and Equipment	18,002
Costs Capitalized Subsequent to Acquisition	12
Cost Amount Carried at Close of Period
Land	630
Buildings and Equipment	18,014
Total	18,644
Accumulated Depreciation	(2,550)
330 South Second Avenue, Minneapolis, MN
Initial Cost to Company
Land	3,990
Buildings and Equipment	18,186
Costs Capitalized Subsequent to Acquisition	9,068
Cost Amount Carried at Close of Period
Land	3,990
Buildings and Equipment	27,254
Total	31,244
Accumulated Depreciation	(3,378)
Rosedale Corporate Plaza, Roseville, MN
Initial Cost to Company
Land	672
Buildings and Equipment	6,045
Costs Capitalized Subsequent to Acquisition	1,444
Cost Amount Carried at Close of Period
Land	672
Buildings and Equipment	7,489
Total	8,161
Accumulated Depreciation	(2,648)
1300 Summit Street, Kansas City, MO
Initial Cost to Company
Land	2,776
Buildings and Equipment	12,070
Costs Capitalized Subsequent to Acquisition	237
Cost Amount Carried at Close of Period
Land	2,776
Buildings and Equipment	12,307
Total	15,083
Accumulated Depreciation	(993)
4241-4300 NE 34th Street, Kansas City, MO
Initial Cost to Company
Land	1,443
Buildings and Equipment	6,193
Costs Capitalized Subsequent to Acquisition	2,734
Cost Amount Carried at Close of Period
Land	1,780
Buildings and Equipment	8,590
Total	10,370
Accumulated Depreciation	(3,436)
1220 Echelon Parkway, Jackson, MS
Initial Cost to Company
Land	440
Buildings and Equipment	25,458
Costs Capitalized Subsequent to Acquisition	48
Cost Amount Carried at Close of Period
Land	440
Buildings and Equipment	25,506
Total	25,946
Accumulated Depreciation	(2,180)
10-12 Celina Avenue, Nashua, NH
Initial Cost to Company
Land	3,000
Buildings and Equipment	14,052
Costs Capitalized Subsequent to Acquisition	371
Cost Amount Carried at Close of Period
Land	3,000
Buildings and Equipment	14,423
Total	17,423
Accumulated Depreciation	(2,269)
50 West State Street, Trenton, NJ
Initial Cost to Company
Land	5,000
Buildings and Equipment	38,203
Costs Capitalized Subsequent to Acquisition	1,347
Cost Amount Carried at Close of Period
Land	5,000
Buildings and Equipment	39,550
Total	44,550
Accumulated Depreciation	(4,901)
435 Montano Boulevard, Albuquerque, NM
Initial Cost to Company
Land	710
Buildings and Equipment	1,651
Costs Capitalized Subsequent to Acquisition	57
Cost Amount Carried at Close of Period
Land	710
Buildings and Equipment	1,708
Total	2,418
Accumulated Depreciation	(228)
138 Delaware Avenue, Buffalo, NY
Initial Cost to Company
Land	4,405
Buildings and Equipment	18,899
Costs Capitalized Subsequent to Acquisition	4,980
Cost Amount Carried at Close of Period
Land	4,485
Buildings and Equipment	23,799
Total	28,284
Accumulated Depreciation	(9,555)
Airline Corporate Center, Colonie, NY
Initial Cost to Company
Land	790
Buildings and Equipment	6,400
Cost Amount Carried at Close of Period
Land	790
Buildings and Equipment	6,400
Total	7,190
Accumulated Depreciation	(560)
5000 Corporate Court, Holtsville, NY
Initial Cost to Company
Land	6,530
Buildings and Equipment	17,711
Costs Capitalized Subsequent to Acquisition	1,770
Cost Amount Carried at Close of Period
Land	6,530
Buildings and Equipment	19,481
Total	26,011
Accumulated Depreciation	(2,033)
305 East 46th Street, New York, NY
Initial Cost to Company
Land	36,800
Buildings and Equipment	66,661
Costs Capitalized Subsequent to Acquisition	2,353
Cost Amount Carried at Close of Period
Land	36,800
Buildings and Equipment	69,014
Total	105,814
Accumulated Depreciation	(7,691)
4600 25th Avenue, Salem, OR
Initial Cost to Company
Land	6,510
Buildings and Equipment	17,973
Costs Capitalized Subsequent to Acquisition	4,069
Cost Amount Carried at Close of Period
Land	6,510
Buildings and Equipment	22,042
Total	28,552
Accumulated Depreciation	(2,463)
Synergy Business Park, Columbia, SC
Initial Cost to Company
Land	1,439
Buildings and Equipment	11,143
Costs Capitalized Subsequent to Acquisition	4,253
Cost Amount Carried at Close of Period
Land	1,439
Buildings and Equipment	15,396
Total	16,835
Accumulated Depreciation	(2,500)
One Memphis Place, Memphis, TN
Initial Cost to Company
Land	1,630
Buildings and Equipment	5,645
Costs Capitalized Subsequent to Acquisition	1,459
Cost Amount Carried at Close of Period
Land	1,630
Buildings and Equipment	7,104
Total	8,734
Accumulated Depreciation	(929)
701 Clay Road, Waco, TX
Initial Cost to Company
Land	2,030
Buildings and Equipment	8,708
Costs Capitalized Subsequent to Acquisition	2,157
Cost Amount Carried at Close of Period
Land	2,060
Buildings and Equipment	10,835
Total	12,895
Accumulated Depreciation	(4,369)
Enterchange at Meadowville, Chester, VA
Initial Cost to Company
Land	1,478
Buildings and Equipment	9,594
Costs Capitalized Subsequent to Acquisition	235
Cost Amount Carried at Close of Period
Land	1,478
Buildings and Equipment	9,829
Total	11,307
Accumulated Depreciation	(566)
3920 Pender Drive, Fairfax, VA
Initial Cost to Company
Land	2,964
Buildings and Equipment	12,840
Costs Capitalized Subsequent to Acquisition	11
Cost Amount Carried at Close of Period
Land	2,963
Buildings and Equipment	12,852
Total	15,815
Accumulated Depreciation	(563)
Pender Business Park, Fairfax, VA
Real estate and accumulated depreciation
Encumbrances	14,160
Initial Cost to Company
Land	2,529
Buildings and Equipment	21,386
Costs Capitalized Subsequent to Acquisition	182
Cost Amount Carried at Close of Period
Land	2,529
Buildings and Equipment	21,568
Total	24,097
Accumulated Depreciation	(1,185)
1759 & 1760 Business Park Drive Reston, VA
Real estate and accumulated depreciation
Encumbrances	83,441
Initial Cost to Company
Land	9,066
Buildings and Equipment	78,658
Costs Capitalized Subsequent to Acquisition	798
Cost Amount Carried at Close of Period
Land	9,066
Buildings and Equipment	79,456
Total	88,522
Accumulated Depreciation	(3,118)
9960 Mayland Drive, Richmond VA
Initial Cost to Company
Land	2,614
Buildings and Equipment	15,930
Costs Capitalized Subsequent to Acquisition	573
Cost Amount Carried at Close of Period
Land	2,614
Buildings and Equipment	16,503
Total	19,117
Accumulated Depreciation	(634)
Aquia Commerce Center, Stafford, VA
Initial Cost to Company
Land	2,090
Buildings and Equipment	7,465
Costs Capitalized Subsequent to Acquisition	193
Cost Amount Carried at Close of Period
Land	2,090
Buildings and Equipment	7,658
Total	9,748
Accumulated Depreciation	(864)
65 Bowdoin Street, S. Burlington, VT
Initial Cost to Company
Land	700
Buildings and Equipment	8,416
Costs Capitalized Subsequent to Acquisition	120
Cost Amount Carried at Close of Period
Land	700
Buildings and Equipment	8,536
Total	9,236
Accumulated Depreciation	(1,223)
840 North Broadway, Everett, WA
Initial Cost to Company
Land	3,360
Buildings and Equipment	15,376
Costs Capitalized Subsequent to Acquisition	1,211
Cost Amount Carried at Close of Period
Land	3,360
Buildings and Equipment	16,587
Total	19,947
Accumulated Depreciation	(1,373)
Stevens Center, Richland, WA
Initial Cost to Company
Land	3,970
Buildings and Equipment	17,035
Costs Capitalized Subsequent to Acquisition	845
Cost Amount Carried at Close of Period
Land	4,042
Buildings and Equipment	17,808
Total	21,850
Accumulated Depreciation	(8,181)
11050 West Liberty Drive, Milwaukee, WI
Initial Cost to Company
Land	945
Buildings and Equipment	4,539
Costs Capitalized Subsequent to Acquisition	132
Cost Amount Carried at Close of Period
Land	945
Buildings and Equipment	4,671
Total	5,616
Accumulated Depreciation	(562)
2029 Stonewall Jackson Drive, Falling Waters, WV
Initial Cost to Company
Land	906
Buildings and Equipment	3,886
Costs Capitalized Subsequent to Acquisition	263
Cost Amount Carried at Close of Period
Land	922
Buildings and Equipment	4,133
Total	5,055
Accumulated Depreciation	(1,900)
5353 Yellowstone Road, Cheyenne, WY
Initial Cost to Company
Land	1,915
Buildings and Equipment	8,217
Costs Capitalized Subsequent to Acquisition	531
Cost Amount Carried at Close of Period
Land	1,950
Buildings and Equipment	8,713
Total	10,663
Accumulated Depreciation	(4,000)
220 E. Bryan Street, Savannah, GA
Initial Cost to Company
Land	950
Buildings and Equipment	2,376
Costs Capitalized Subsequent to Acquisition	20
Cost Amount Carried at Close of Period
Land	950
Buildings and Equipment	2,396
Total	3,346
Accumulated Depreciation	(275)
5600 Columbia Pike, Falls Church, VA
Initial Cost to Company
Land	3,519
Buildings and Equipment	15,098
Costs Capitalized Subsequent to Acquisition	4,065
Impairment/ Writedowns	(10,423)
Cost Amount Carried at Close of Period
Land	3,044
Buildings and Equipment	9,215
Total	12,259
Total Real Estate Assets
Real estate and accumulated depreciation
Encumbrances	136,643
Initial Cost to Company
Land	256,615
Buildings and Equipment	1,337,531
Costs Capitalized Subsequent to Acquisition	128,016
Impairment/ Writedowns	(10,423)
Cost Amount Carried at Close of Period
Land	257,052
Buildings and Equipment	1,454,685
Total	1,711,737
Accumulated Depreciation	$ (256,154)