Like-Kind Exchanges (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Investment Holdings [Line Items]
Prepaid expenses and other current assets	$ 482,000	$ 4,385,000
Deferred tax and other assets	1,134,000	2,664,000
Like-Kind Intermediaries
Investment Holdings [Line Items]
Prepaid expenses and other current assets	0	850,000
Deferred tax and other assets	$ 0	$ 1,900,000