Fair Value Measurements (Narrative) (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) investment	Dec. 31, 2018 USD ($) investment
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate fund investments	$ 322,858	$ 318,758
Real estate fund investments
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Number of investments held by fund (investment) | investment	4	4
Excess of fair value over cost	$ 6,706
Level 3 | Real estate fund investments
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate fund investments	$ 322,858