Impairment of Real Estate and Impairment of Unconsolidated Entities (Detail Textual)	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) a	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) a	Sep. 30, 2014 USD ($)
Impairment of Real Estate and Impairment of Unconsolidated Entities [Line Items]
Impairment of Real Estate	$ 425,463,000	$ 966,000	$ 425,463,000	$ 130,795,000
Impairment of Unconsolidated Entities	$ 1,384,000	0	$ 1,384,000	0
Westchester's Ridge Hill [Member]
Impairment of Real Estate and Impairment of Unconsolidated Entities [Line Items]
Previous probability of a near-term sale	10.00%		10.00%
Impairment of Real Estate	$ 372,587,000	0	$ 372,587,000	0
Illinois Science and Technology Park [Member]
Impairment of Real Estate and Impairment of Unconsolidated Entities [Line Items]
Impairment of Real Estate	26,246,000	0	26,246,000	0
Land inventory [Member]
Impairment of Real Estate and Impairment of Unconsolidated Entities [Line Items]
Impairment of Real Estate	$ 16,307,000	$ 0	$ 16,307,000	$ 0
Area of Land | a	13.5		13.5
Three consolidated regional malls [Member]
Impairment of Real Estate and Impairment of Unconsolidated Entities [Line Items]
Number of regional malls company pursuing partial sale through joint venture		3
Likelihood of a near-term partial sale	70.00%		70.00%
Probably of a long-term hold for each asset	30.00%		30.00%