Real Estate - Real Estate Under Development and Construction in Progress - Additional Information (Details) - property	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Number of unconsolidated project in development			1
Fund III
Property, Plant and Equipment [Line Items]
Number of projects put into service during period	1
Core Portfolio
Property, Plant and Equipment [Line Items]
Number of projects put into service during period	1	1