Debt - Summary of Consolidated Indebtedness (Parenthetical) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,491,869	$ 1,438,386
Variable-rate debt that subject to interest cap agreements
Debt Instrument [Line Items]
Long-term debt, gross	143,800	141,100
Total Debt - Variable Rate Debt Fixed with Interest Rate Swap Agreements
Debt Instrument [Line Items]
Long-term debt, gross	$ 575,500	$ 504,000
Minimum | Core Portfolio | Variable Rate Debt | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.70%
Minimum | Core Portfolio | Unsecured Notes Payable | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.25%
Minimum | Core Portfolio | Unsecured Line of Credit | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.35%
Maximum | Core Portfolio | Variable Rate Debt | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.90%
Fund II | Fund Portfolio | Variable Rate Debt | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.39%
Fund III | Minimum | Fund Portfolio | Variable Rate Debt | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	2.65%
Fund III | Maximum | Fund Portfolio | Variable Rate Debt | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	4.65%
Fund IV | Minimum | Fund Portfolio | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.60%
Fund IV | Maximum | Fund Portfolio | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	3.95%
Fund V | Minimum | Fund Portfolio | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	2.15%
Fund V | Maximum | Fund Portfolio | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	2.25%