Credit Facilities (Narrative) (Details)		3 Months Ended
	Mar. 13, 2019 USD ($) property	Mar. 31, 2020 property	Mar. 31, 2020 USD ($)	Mar. 31, 2020	Mar. 31, 2020 encumbered_property	Dec. 31, 2019 USD ($)	Nov. 01, 2019 USD ($) instrument	Apr. 15, 2019	Oct. 31, 2016 instrument
Line of Credit Facility [Line Items]
Total real estate investments, at cost			$ 2,638,245,000			$ 2,481,067,000
Line of credit facility, current borrowing capacity base percent		55.00%
Number of properties		200			113
Effective interest rate				3.80%		4.14%
Outstanding balance			700,546,000			$ 605,269,000
Notional amount							$ 88,700,000
Unencumbered Properties
Line of Credit Facility [Line Items]
Total real estate investments, at cost			302,100,000
Secured Debt
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 630,000,000.0
Unused borrowing capacity			37,400,000
Credit Facility
Line of Credit Facility [Line Items]
Outstanding balance			341,224,000			245,947,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Number of properties | property	65
Minimum | Secured Debt
Line of Credit Facility [Line Items]
Line of credit, increase	$ 370,000,000.0
Maximum | Secured Debt
Line of Credit Facility [Line Items]
Line of credit, increase	1,000,000,000.0
Fannie Mae Master Credit Facilities
Line of Credit Facility [Line Items]
Total real estate investments, at cost			500,000,000
Fannie Mae Master Credit Facilities | Credit Facility
Line of Credit Facility [Line Items]
Number of properties | property		22
Outstanding balance			359,322,000			$ 359,322,000
New Credit Facilities
Line of Credit Facility [Line Items]
Total real estate investments, at cost			1,300,000,000
Long-term debt, covenant requirements, distribution in excess of modified funds from operations, percent		95.00%
New Credit Facilities | Minimum
Line of Credit Facility [Line Items]
Long-term debt, covenant requirements, amount			50,000,000.0
Mortgages
Line of Credit Facility [Line Items]
Total real estate investments, at cost			900,000,000
Effective interest rate				3.91%		3.90%
Long-term debt			542,077,000			$ 528,284,000
Credit Facility | Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 480,000,000.0
Number of properties | property		91
Effective interest rate				3.27%		4.08%
Outstanding balance			195,618,000			$ 100,618,000
Credit Facility | Revolving Credit Facility | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate, base rate calculation	1.00%
Credit Facility | Revolving Credit Facility | Federal Funds Effective Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
Credit Facility | Minimum | Revolving Credit Facility | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.60%
Credit Facility | Minimum | Revolving Credit Facility | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.35%
Credit Facility | Maximum | Revolving Credit Facility | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate	2.20%
Credit Facility | Maximum | Revolving Credit Facility | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.95%
Term Loan
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 150,000,000.0
Effective interest rate				3.63%
Long-term debt			150,000,000.0
Term Loan | Credit Facility
Line of Credit Facility [Line Items]
Effective interest rate						4.05%
Outstanding balance			$ 150,000,000			$ 150,000,000
Term Loan | Minimum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.55%
Term Loan | Minimum | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.30%
Term Loan | Maximum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate	2.15%
Term Loan | Maximum | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.90%
Term Loan | New Credit Facilities
Line of Credit Facility [Line Items]
Weighted average rate								2.30%
Interest rate caps | Fannie Credit Facility
Line of Credit Facility [Line Items]
Number of instruments | instrument							2		2
Interest rate cap									3500.00%