Provision for Impairment of Investment Properties - Impairment Indicators (Details) - property	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Impaired Long-Lived Assets Held and Used [Line Items]
Number of properties for which indicators of impairment were identified	1	0	6
Number of properties for which an impairment charge was recorded	1		1
Number of properties held for sale with impairment indicators but not impaired	0		1
Remaining properties for which indicators of impairment were identified but no impairment was considered necessary	0		4
Weighted average percentage by which projected undiscounted cash flows exceeded carrying value for each of the remaining properties			14.00%
Number of properties with impairment indicators which were subsequently sold			5