Real Estate and Accumulated Depreciation (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	$ 280,576
Initial Cost to Registrant, Land	36,148
Initial Cost to Registrant, Building and Improvements	366,805
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	19,036
Land	43,252
Building and Improvements	378,737
Total	421,989
Net of accumulated depreciation	51,553
Office [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	231,029
Initial Cost to Registrant, Land	26,054
Initial Cost to Registrant, Building and Improvements	263,834
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	30,482
Land	31,254
Building and Improvements	289,116
Total	320,370
Net of accumulated depreciation	40,044
Office [Member] | Orlando [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	37,580
Initial Cost to Registrant, Land
Initial Cost to Registrant, Building and Improvements	17,248
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	42
Land
Building and Improvements	17,290
Total	17,290
Net of accumulated depreciation	3,512
Date Acquired	Nov. 01, 2004
Life	40 years
Office [Member] | Plantation [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	10,811
Initial Cost to Registrant, Land
Initial Cost to Registrant, Building and Improvements	8,915
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	4,020
Land	4,000
Building and Improvements	8,935
Total	12,935
Net of accumulated depreciation	1,815
Date Acquired	Nov. 01, 2004
Life	40 years
Office [Member] | Chicago 1 [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	20,200
Initial Cost to Registrant, Land
Initial Cost to Registrant, Building and Improvements	23,635
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	2,388
Land
Building and Improvements	26,023
Total	26,023
Net of accumulated depreciation	4,986
Date Acquired	Oct. 01, 2005
Life	40 years
Office [Member] | Amherst [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	15,225
Initial Cost to Registrant, Land	1,591
Initial Cost to Registrant, Building and Improvements	18,027
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	7
Land	1,591
Building and Improvements	18,034
Total	19,625
Net of accumulated depreciation	3,436
Date Acquired	May 01, 2005
Life	40 years
Office [Member] | Andover [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances
Initial Cost to Registrant, Land
Initial Cost to Registrant, Building and Improvements	7,611
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	718
Land	1,200
Building and Improvements	7,129
Total	8,329
Net of accumulated depreciation	1,417
Date Acquired	Dec. 01, 2005
Life	40 years
Office [Member] | South Burlington [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances
Initial Cost to Registrant, Land
Initial Cost to Registrant, Building and Improvements	3,099
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	309
Land
Building and Improvements	3,408
Total	3,408
Net of accumulated depreciation	578
Date Acquired	Dec. 01, 2005
Life	40 years
Office [Member] | Chicago 2 [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	8,700
Initial Cost to Registrant, Land	1,149
Initial Cost to Registrant, Building and Improvements	9,989
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	5,619
Land	1,149
Building and Improvements	15,608
Total	16,757
Net of accumulated depreciation	2,037
Date Acquired	Oct. 01, 2007
Life	40 years
Office [Member] | Houston [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	52,052
Initial Cost to Registrant, Land	7,075
Initial Cost to Registrant, Building and Improvements	62,468
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements
Land	7,075
Building and Improvements	62,468
Total	69,543
Net of accumulated depreciation	12,624
Date Acquired	Jan. 01, 2005
Life	40 years
Office [Member] | Lisle 1 [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	5,752
Initial Cost to Registrant, Land	3,774
Initial Cost to Registrant, Building and Improvements	16,371
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	2,406
Land	3,774
Building and Improvements	18,777
Total	22,551
Net of accumulated depreciation	3,167
Date Acquired	Feb. 01, 2006
Life	40 years
Office [Member] | Lisle 2 [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances
Initial Cost to Registrant, Land	2,361
Initial Cost to Registrant, Building and Improvements	6,298
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	(2,366)
Land	2,361
Building and Improvements	3,932
Total	6,293
Net of accumulated depreciation	1,007
Date Acquired	Feb. 01, 2006
Life	40 years
Office [Member] | Lisle 3 [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	5,543
Initial Cost to Registrant, Land	780
Initial Cost to Registrant, Building and Improvements	2,803
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	673
Land	780
Building and Improvements	3,476
Total	4,256
Net of accumulated depreciation	540
Date Acquired	Feb. 01, 2006
Life	40 years
Office [Member] | Phoenix [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances
Initial Cost to Registrant, Land	801
Initial Cost to Registrant, Building and Improvements	7,387
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	3,858
Land	801
Building and Improvements	11,245
Total	12,046
Net of accumulated depreciation	1,446
Date Acquired	Aug. 01, 2010
Life	40 years
Office [Member] | Englewood 1 [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances
Initial Cost to Registrant, Land	2,580
Initial Cost to Registrant, Building and Improvements	5,403
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	3,242
Land	2,580
Building and Improvements	8,645
Total	11,225
Net of accumulated depreciation	806
Date Acquired	Nov. 01, 2010
Life	40 years
Office [Member] | Englewood 2 [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances
Initial Cost to Registrant, Land	1,829
Initial Cost to Registrant, Building and Improvements	5,612
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	692
Land	1,829
Building and Improvements	6,304
Total	8,133
Net of accumulated depreciation	579
Date Acquired	Dec. 01, 2010
Life	40 years
Office [Member] | New York [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	51,982
Initial Cost to Registrant, Land
Initial Cost to Registrant, Building and Improvements	52,778
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	7,598
Land
Building and Improvements	60,376
Total	60,376
Net of accumulated depreciation	1,937
Date Acquired	Nov. 01, 2011
Life	40 years
Office [Member] | Cerritos [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	23,184
Initial Cost to Registrant, Land	4,114
Initial Cost to Registrant, Building and Improvements	16,190
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	1,276
Land	4,114
Building and Improvements	17,466
Total	21,580
Net of accumulated depreciation	157
Date Acquired	Oct. 01, 2012
Life	40 years
Retail [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances
Initial Cost to Registrant, Land
Initial Cost to Registrant, Building and Improvements	14,119
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	(188)
Land	1,904
Building and Improvements	12,027
Total	13,931
Net of accumulated depreciation	2,895
Retail [Member] | Atlanta [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances
Initial Cost to Registrant, Land
Initial Cost to Registrant, Building and Improvements	4,633
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	(1,733)
Land
Building and Improvements	2,900
Total	2,900
Net of accumulated depreciation	942
Date Acquired	Nov. 01, 2004
Life	40 years
Retail [Member] | Louisville [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances
Initial Cost to Registrant, Land
Initial Cost to Registrant, Building and Improvements	2,722
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	376
Land	373
Building and Improvements	2,725
Total	3,098
Net of accumulated depreciation	554
Date Acquired	Nov. 01, 2004
Life	40 years
Retail [Member] | Greensboro [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances
Initial Cost to Registrant, Land
Initial Cost to Registrant, Building and Improvements	3,797
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	4
Land
Building and Improvements	3,801
Total	3,801
Net of accumulated depreciation	772
Date Acquired	Nov. 01, 2004
Life	40 years
Retail [Member] | Denton [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances
Initial Cost to Registrant, Land
Initial Cost to Registrant, Building and Improvements	1,574
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	546
Land	915
Building and Improvements	1,205
Total	2,120
Net of accumulated depreciation	344
Date Acquired	Nov. 01, 2004
Life	40 years
Retail [Member] | Seabrook [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances
Initial Cost to Registrant, Land
Initial Cost to Registrant, Building and Improvements	1,393
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	619
Land	616
Building and Improvements	1,396
Total	2,012
Net of accumulated depreciation	283
Date Acquired	Nov. 01, 2004
Life	40 years
Warehouse [Member] | Jacksonville [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances
Initial Cost to Registrant, Land	2,166
Initial Cost to Registrant, Building and Improvements	8,684
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	1,771
Land	2,166
Building and Improvements	10,455
Total	12,621
Net of accumulated depreciation	2,195
Date Acquired	Nov. 01, 2004
Life	40 years
Mixed use [Member] | Churchill [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances
Initial Cost to Registrant, Land
Initial Cost to Registrant, Building and Improvements	23,834
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	(14,129)
Land
Building and Improvements	9,705
Total	9,705
Net of accumulated depreciation	3,960
Date Acquired	Nov. 01, 2004
Life	40 years
Multi-family [Member] | Greensboro [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	15,139
Initial Cost to Registrant, Land	1,961
Initial Cost to Registrant, Building and Improvements	16,482
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements
Land	1,961
Building and Improvements	16,482
Total	18,443
Net of accumulated depreciation	109
Date Acquired	Nov. 01, 2012
Life	40 years
Multi-family [Member] | Meriden [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	21,000
Initial Cost to Registrant, Land	2,887
Initial Cost to Registrant, Building and Improvements	22,367
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	299
Land	2,887
Building and Improvements	22,666
Total	25,553
Net of accumulated depreciation	1,880
Date Acquired	Oct. 01, 2010
Life	40 years
Multi-family [Member] | Memphis [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	13,408
Initial Cost to Registrant, Land	3,080
Initial Cost to Registrant, Building and Improvements	17,485
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	801
Land	3,080
Building and Improvements	18,286
Total	21,366
Net of accumulated depreciation	470
Date Acquired	Apr. 01, 2012
Life	40 years
Other [Member]
Real Estate and Accumulated Depreciation [Line Items]
Mortgage Encumbrances	49,547
Initial Cost to Registrant, Land	10,094
Initial Cost to Registrant, Building and Improvements	88,852
Cost capitalized/(impaired) subsequent to acquisition Land/Building and Improvements	(11,258)
Land	10,094
Building and Improvements	77,594
Total	87,688
Net of accumulated depreciation	$ 8,614