Income Taxes - Expense (benefit) reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Taxes
Statutory federal income tax rate (as a percent)			21.00%	35.00%	35.00%
Tax at the statutory federal rate	$ 420	$ 3,005	$ 7,402	$ 11,167
State income taxes (net of federal benefit)	88	301	1,532	1,117
2017 qualified timber gains at the federal statutory rate of 23.8% (1)			(524)
Decrease in valuation allowance	(3,480)	(250)	(4,931)	(846)
Change in federal AMT credit carryforward	(511)		(511)
Other		(413)	(153)	(607)
Income tax (benefit) expense	$ (3,483)	$ 2,643	$ 2,815	$ 10,831
Timber statutory federal rate				23.80%