Fair Value Measurements (Narrative) (Details) $ in Thousands			6 Months Ended	12 Months Ended
	Jan. 11, 2018	Jan. 04, 2018	Jun. 30, 2018 USD ($) investment	Dec. 31, 2017 USD ($) investment
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Number of investments held by fund | investment				5
Real estate fund investments			$ 373,039	$ 354,804
Real estate fund investments
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Number of investments held by fund | investment			4
Recurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate fund investments			$ 373,039	354,804
Level 3 | Recurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate fund investments				$ 354,804
Redeemable Preferred Stock | Series G Preferred Stock
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Preferred stock, dividend rate, percentage	6.625%	6.625%
Redeemable Preferred Stock | Series I Preferred Stock
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Preferred stock, dividend rate, percentage	6.625%	6.625%
Discounted Cash Flow Technique | Real estate fund investments
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Excess of fair value over cost			47,475
Discounted Cash Flow Technique | Level 3 | Real estate fund investments
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate fund investments			$ 373,039
Discounted Cash Flow Technique | Level 3 | Recurring | Minimum
Fair Value Inputs [Abstract]
Anticipated holding period of investments			6 months
Discounted Cash Flow Technique | Level 3 | Recurring | Maximum
Fair Value Inputs [Abstract]
Anticipated holding period of investments			4 years 6 months