Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Real Estate [Line Items]
Required minimum percentage distribution of of ordinary taxable income to stockholders to qualify as a REIT			90.00%
Income Tax Expense (Benefit)	$ 1,600		$ (1,604)	$ (47)	$ 2,726
Maximum
Real Estate [Line Items]
Income Tax Expense (Benefit)		$ 100
AJS and Holstenplatz
Real Estate [Line Items]
Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Amount	800
Foreign Tax Authority
Real Estate [Line Items]
Effective Income Tax Rate Reconciliation, Tax Settlement, Foreign, Amount	$ 800