Debt - Summary of Consolidated Indebtedness (Parenthetical) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,818,679	$ 1,717,928
Variable-rate debt that subject to interest cap agreements
Debt Instrument [Line Items]
Long-term debt, gross	144,500	143,300
Total Debt - Variable Rate Debt Fixed with Interest Rate Swap Agreements
Debt Instrument [Line Items]
Long-term debt, gross	1,029,100	948,800
Core Portfolio | Interest Rate Swaps
Debt Instrument [Line Items]
Long-term debt, gross	$ 70,200
Minimum | Core Portfolio | Variable Rate Debt | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
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 [Member] | 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
Debt Instrument [Line Items]
Long-term debt, gross	$ 25,397	$ 24,225
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	2.75%
Fund III | Maximum | Fund Portfolio | Variable Rate Debt | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	3.10%
Fund IV | Minimum | Fund Portfolio | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%
Fund IV | Maximum | Fund Portfolio | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	2.25%
Fund V | Minimum | Fund Portfolio | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
Fund V | Maximum | Fund Portfolio | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	2.20%