Summary of Significant Accounting Policies - Real Estate Investments (Details)	12 Months Ended
	Dec. 31, 2017
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment estimated useful life	40 years
Minimum [Member] | Building Fixtures and Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment estimated useful life	5 years
Maximum [Member] | Building Fixtures and Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment estimated useful life	15 years