Fair Value Measurements (Narrative) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) investment	Dec. 31, 2016 USD ($)
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate fund investments	$ 351,750	$ 462,132
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	$ 351,750
Excess of fair value over cost	95,136
Recurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate fund investments	351,750	462,132
Excess of fair value over cost	95,136
Level 3 | Real estate fund investments
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate fund investments	351,750
Level 3 | Recurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate fund investments	$ 351,750	$ 462,132
Level 3 | Recurring | Minimum
Fair Value Inputs [Abstract]
Anticipated holding period of investments	9 months 18 days
Level 3 | Recurring | Maximum
Fair Value Inputs [Abstract]
Anticipated holding period of investments	3 years 3 months 18 days