Debt - Unsecured Revolving Lines of Credit - Additional Information (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018	Feb. 20, 2018
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 300,000,000
Letters of credit, outstanding amount	$ 27,700,000	$ 19,700,000
Revolving Credit Facility | Unsecured Debt
Debt Instrument [Line Items]
Maximum borrowing capacity			150,000,000
Core Portfolio | Revolving Credit Facility | Unsecured Debt
Debt Instrument [Line Items]
Debt available balance	120,700,000	137,700,000
Maximum borrowing capacity	150,000,000		$ 150,000,000
Credit facility amount outstanding	9,000,000	0
Letters of credit, outstanding amount	$ 20,300,000	$ 12,300,000