Fair Value Measurements (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) investment	Dec. 31, 2016 USD ($) investment
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Number of investments held by fund | investment	5	6
Real estate fund investments	$ 354,804	$ 462,132
Real estate fund investments
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate fund investments	354,804
Excess of fair value over cost	98,189
Recurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate fund investments	354,804	462,132
Recurring | Level 3
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate fund investments	$ 354,804	$ 462,132
Recurring | Level 3 | Minimum
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Anticipated holding period of investments	3 months 18 days
Recurring | Level 3 | Maximum
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Anticipated holding period of investments	5 years
Recurring | Level 3 | Real estate fund investments
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Number of investments held by fund | investment	5
Real estate fund investments	$ 354,804
Excess of fair value over cost	$ 98,189
Redeemable Preferred Stock | Series G
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Preferred stock dividend rate	6.625%
Redeemable Preferred Stock | Series I
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Preferred stock dividend rate	6.625%