Significant Accounting Policies - Investment in Real Estate (Details) - Real estate	12 Months Ended
	Dec. 31, 2018
Land improvements | Minimum
Property, plant and equipment
Estimated useful life (in years)	15 years
Land improvements | Maximum
Property, plant and equipment
Estimated useful life (in years)	20 years
Buildings | Minimum
Property, plant and equipment
Estimated useful life (in years)	20 years
Buildings | Maximum
Property, plant and equipment
Estimated useful life (in years)	40 years
Building improvements | Minimum
Property, plant and equipment
Estimated useful life (in years)	5 years
Building improvements | Maximum
Property, plant and equipment
Estimated useful life (in years)	25 years