Variable Interest Entities (Narrative) (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) property joint_venture	Dec. 31, 2018 property joint_venture
Variable Interest Entity [Line Items]
Number of VIE real estate joint ventures guaranteed by company	1	1
Consolidated Variable Interest Entities [Member]
Variable Interest Entity [Line Items]
Number of VIE real estate joint ventures	9	9
Number of real estate properties | property	21	21
Unconsolidated Variable Interest Entities [Member]
Variable Interest Entity [Line Items]
Number of VIE real estate joint ventures	2	2
Number of joint venture arrangements	1
Variable interest entity, nonconsolidated, additional future funding | $	$ 48