Debt - Unsecured Lines of Credit (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Letters of credit, outstanding amount	$ 19,700,000	$ 11,300,000
Core Portfolio | Revolving Credit Facility | Unsecured Debt
Debt Instrument [Line Items]
Debt available balance	96,200,000	138,700,000
Maximum borrowing capacity	150,000,000.0
Credit facility amount outstanding	41,500,000	0
Letters of credit, outstanding amount	$ 12,300,000	$ 11,300,000