Mortgage Debt - Mortgage Debt, Additional Information (Details) - acquisition	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Number of acquisitions partially financed with mortgages	4
Mortgages [Member]
Debt Instrument [Line Items]
Weighted-average interest rate of investment debt	3.42%	3.35%
Percent of property level debt with fixed rate	74.00%	74.00%
Percent of property level debt with floating rate and interest caps	14.00%	8.00%
Percent of property level debt with floating rate, without interest caps	11.00%	18.00%
Weighted average cap strike price	2.51%