Merger with Deltic (Narrative) (Details) $ in Thousands				3 Months Ended	9 Months Ended
		Feb. 20, 2018 USD ($) a Sawmill	Feb. 19, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Business Acquisition [Line Items]
Merger-related costs	[1]			$ 972	$ 21,245
Deltic [Member]
Business Acquisition [Line Items]
Acres of timberland | a		530,000
Number of sawmills | Sawmill		2
Total assets acquired		$ 1,400,000
Equity consideration transferred		1,100,000
Liabilities assumed		$ 300,000
Merger-related costs				1,000	21,200
Merger related costs, professional fees					11,500
Restructuring costs, termination benefits					9,700
Non-recurring merger-related costs			$ 5,400	$ 1,000	$ 26,700

[1] For integration and restructuring costs related to the merger with Deltic see Note 3: Merger with Deltic.