Investments in Unconsolidated Companies (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) entities	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Equity Method Investments [Line Items]
Equity in earnings (loss) of unconsolidated joint ventures	$ 63,310	$ 47,403	$ (3,304)
Impairment charges	$ 4,481	$ 18,018	22,932
Number of unconsolidated joint ventures in which equity interests held | entities	9
Equity Method Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Impairment charges			30,000
Number of Impaired Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Number of unconsolidated joint ventures in which equity interests held | entities	3
Linden, New Jersey [Member]
Schedule of Equity Method Investments [Line Items]
Impairment charges			$ 19,500
Medical Office Portfolio Sale [Member]
Schedule of Equity Method Investments [Line Items]
Equity in earnings (loss) of unconsolidated joint ventures	$ 47,500
Number of unconsolidated joint ventures in which equity interests held | entities	2