Debt - Summary of Consolidated Indebtedness (Parenthetical) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,834,375	$ 1,717,928
Variable-rate debt that subject to interest cap agreements
Debt Instrument [Line Items]
Long-term debt, gross	143,600	$ 143,300
LIBOR | Mortgages
Debt Instrument [Line Items]
Basis spread on variable rate		1.70%
Total Debt - Variable Rate Debt Fixed with Interest Rate Swap Agreements
Debt Instrument [Line Items]
Long-term debt, gross	1,038,800	$ 948,800
Core Portfolio | Interest Rate Swaps
Debt Instrument [Line Items]
Long-term debt, gross	70,200
Core Portfolio | Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 256,927	257,735
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	$ 24,517	24,225
Fund II | Fund Portfolio | Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 243,524	$ 243,298
Fund II | Fund Portfolio | Prime Rate | Mortgages
Debt Instrument [Line Items]
Interest rate, stated percentage	0.02%
Fund II | Fund Portfolio | Variable Rate Debt | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.39%
Fund II | Fund Portfolio | Variable Rate Debt | LIBOR | Mortgages
Debt Instrument [Line Items]
Basis spread on variable rate	3.00%	3.00%
Fund III | Fund Portfolio | Variable Rate Debt | Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 74,952	$ 74,554
Fund III | Minimum | Fund Portfolio | Variable Rate Debt | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	2.75%
Fund III | Minimum | Fund Portfolio | Variable Rate Debt | LIBOR | Mortgages
Debt Instrument [Line Items]
Basis spread on variable rate	2.75%	2.75%
Fund III | Maximum | Fund Portfolio | Variable Rate Debt | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	3.10%
Fund III | Maximum | Fund Portfolio | Variable Rate Debt | LIBOR | Mortgages
Debt Instrument [Line Items]
Basis spread on variable rate	3.10%	3.10%
Fund IV | Fund Portfolio | Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 267,322	$ 267,903
Fund IV | Fund Portfolio | Variable Rate Debt | Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 179,925	$ 157,015
Fund IV | Minimum | Fund Portfolio | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%
Fund IV | Minimum | Fund Portfolio | Variable Rate Debt | LIBOR | Mortgages
Debt Instrument [Line Items]
Basis spread on variable rate	1.60%	1.60%
Fund IV | Maximum | Fund Portfolio | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	2.25%
Fund IV | Maximum | Fund Portfolio | Variable Rate Debt | LIBOR | Mortgages
Debt Instrument [Line Items]
Basis spread on variable rate	3.40%	3.40%
Fund V | Fund Portfolio | Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 335,980	$ 336,013
Fund V | Fund Portfolio | Variable Rate Debt | Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,387	$ 1,387
Fund V | Minimum | Fund Portfolio | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
Fund V | Minimum | Fund Portfolio | Variable Rate Debt | LIBOR | Mortgages
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%	1.50%
Fund V | Maximum | Fund Portfolio | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	2.20%
Fund V | Maximum | Fund Portfolio | Variable Rate Debt | LIBOR | Mortgages
Debt Instrument [Line Items]
Basis spread on variable rate	2.20%	2.20%