Debt - Unsecured Revolving Lines of Credit (Details) - USD ($)	Mar. 31, 2018	Feb. 20, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 300,000,000.0
Letters of credit, outstanding amount	$ 19,700,000		$ 19,700,000
Core Portfolio | Revolving Credit Facility | Unsecured Debt
Debt Instrument [Line Items]
Debt available balance	123,700,000		96,200,000
Maximum borrowing capacity	150,000,000.0
Credit facility amount outstanding	14,000,000		41,500,000
Letters of credit, outstanding amount	$ 12,300,000		$ 12,300,000