Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease term				48 months
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606 | Land
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues	$ 73,000	$ 1,112,000
Profit from the sale of land	9,000	1,017,000
Accounting Standards Update 2016-02 | Subsequent Event
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liability			$ 52,000,000
Operating lease right-of-use asset			$ 52,000
Discount rate for future annual cash lease payments			4.11%
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Corporate expenses	$ (428,000)	$ (739,000)