Merger with Deltic (Narrative) (Details)				3 Months Ended		6 Months Ended
		Feb. 20, 2018 a Sawmill shares	Feb. 19, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) shares	Jun. 30, 2017 USD ($)	Mar. 31, 2018 USD ($)
Business Acquisition [Line Items]
Number of share exchaged under agreement	[1]	180.00%
Consulting agreement period						2 years
Business combination recorded merger costs								$ 1,850,000
Merger-related costs	[2]			$ 1,018,000		$ 20,273,000
Number of shares issued upon merger | shares		21,981,128
Mineral rights				6,236,000		6,236,000
Other long-term assets				3,472,000		3,472,000		2,010,000
Property, plant and equipment				(261,331,000)		(261,331,000)		(265,901,000)
Timber and timberlands				(1,058,000,000)		(1,058,000,000)		$ (1,060,000,000)
Depreciation, depletion and amortization				21,605,000	$ 6,641,000	34,240,000	$ 13,343,000
Measurement Period Adjustment [Member]
Business Acquisition [Line Items]
Mineral rights				6,236,000		6,236,000
Other long-term assets				1,462,000		1,462,000
Property, plant and equipment				4,570,000		4,570,000
Timber and timberlands				2,000,000		2,000,000
Depreciation, depletion and amortization				200,000
Deltic [Member]
Business Acquisition [Line Items]
Acres of timberland | a		530,000
Number of sawmills | Sawmill		2
Area of diversified timberland base after merger | a		2,000,000
Number of share exchaged under agreement		180.00%
Non-recurring merger-related costs			$ 5,400,000	$ 1,000,000	$ 0	$ 25,700,000	$ 0
Number of shares issued upon merger | shares						22,000,000
Stock dividend to be issued in connection with earnings and profits distribution. | shares						3,900,000
Earnings and profits dividend rate or special distribution percentage of stock		80.00%
Earnings and profits dividend rate or special distribution percentage of cash		20.00%
Deltic [Member] | Arkansas [Member]
Business Acquisition [Line Items]
Area of diversified timberland base after merger | a		930,000

[1] Exchange ratio per the merger agreement.

[2] For integration and restructuring costs related to the merger with Deltic see Note 14: Merger, Integration and Other Costs.