Income Taxes - Tax credit carryforward (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Tax credit carryforward
Income tax receivable	$ 3,395	$ 8,371
Federal
Tax credit carryforward
Increase in alternative minimum tax credit credit carryforward	1,000
ATM credit carryforward | Federal
Tax credit carryforward
Income tax receivable	3,400	$ 8,400
Amount of income tax payable applied to income tax receivable	6,000
ATM credit carryforward, Qualified Timber Gain preferential rate | Federal
Tax credit carryforward
Increase in alternative minimum tax credit credit carryforward	500
ATM credit carryforward, Actual deductible expenses | Federal
Tax credit carryforward
Increase in alternative minimum tax credit credit carryforward	$ 500