Fair Value of Financial Instruments and Investments - Summary of Effect of Movements in Interest Rate Markets (Detail) (USD $)	9 Months Ended		12 Months Ended
	Sep. 30, 2012		Dec. 31, 2011
Non-qualifying Interest Rate Swap on Thames Valley Drive Retail Park debt [Member]
Derivative [Line Items]
Notional Amount	$ 9,107,000	[1]	$ 8,762,000	[1]
Maturity Date	May 30, 2013	[1]	May 30, 2013	[1]
Effects of change of negative hundred basis point in interest rates on fair value	(24)	[1]
Effects of change of negative fifty basis point in interest rates on fair value	(23)	[1]
Effects of change of positive fifty basis point in interest rates on fair value	23	[1]
Effects of change of positive hundred basis point in interest rates on fair value	46	[1]
Non-qualifying Interest Rate Swap on Albion Mills debt [Member]
Derivative [Line Items]
Notional Amount	9,227,000	[1]	8,877,000	[1]
Maturity Date	Oct. 10, 2013	[1]	Oct. 10, 2013	[1]
Effects of change of negative hundred basis point in interest rates on fair value	(48)	[1]
Effects of change of negative fifty basis point in interest rates on fair value	(46)	[1]
Effects of change of positive fifty basis point in interest rates on fair value	46	[1]
Effects of change of positive hundred basis point in interest rates on fair value	92	[1]
Qualifying Interest Rate Swap on Maskew Retail Park debt [Member]
Derivative [Line Items]
Notional Amount	22,566,000	[1]	21,710,000	[1]
Maturity Date	Aug. 10, 2014	[1]	Aug. 10, 2014	[1]
Effects of change of negative hundred basis point in interest rates on fair value	(220)	[1]
Effects of change of negative fifty basis point in interest rates on fair value	(204)	[1]
Effects of change of positive fifty basis point in interest rates on fair value	202	[1]
Effects of change of positive hundred basis point in interest rates on fair value	401	[1]
Qualifying Interest Rate Swap on Pacific Corporate Park Debt [Member]
Derivative [Line Items]
Notional Amount	79,750,000		82,000,000
Maturity Date	Dec. 07, 2017		Dec. 07, 2017
Effects of change of negative hundred basis point in interest rates on fair value	(2,514)
Effects of change of negative fifty basis point in interest rates on fair value	(1,783)
Effects of change of positive fifty basis point in interest rates on fair value	1,772
Effects of change of positive hundred basis point in interest rates on fair value	3,728
Qualifying Interest Rate Swap on 100 Kimball Drive debt [Member]
Derivative [Line Items]
Notional Amount	32,023,000		32,521,000
Maturity Date	Mar. 01, 2021		Mar. 01, 2021
Effects of change of negative hundred basis point in interest rates on fair value	(2,133)
Effects of change of negative fifty basis point in interest rates on fair value	(1,160)
Effects of change of positive fifty basis point in interest rates on fair value	1,122
Effects of change of positive hundred basis point in interest rates on fair value	2,313
Qualifying Interest Rate Swap on Kings Mountain III debt [Member]
Derivative [Line Items]
Notional Amount	11,401,000		11,595,000
Maturity Date	Jul. 01, 2018		Jul. 01, 2018
Effects of change of negative hundred basis point in interest rates on fair value	(477)
Effects of change of negative fifty basis point in interest rates on fair value	(289)
Effects of change of positive fifty basis point in interest rates on fair value	285
Effects of change of positive hundred basis point in interest rates on fair value	$ 598

[1] Based on three month GBP-based LIBOR BBA Index with variable rate reset dates every 90 days during the term of the swaps.