Summary of Significant Accounting Policies (Narrative) (Details) $ in Thousands, a in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) a
Principle Accounting Policies [Line Items]
Timber And Timberlands Acres Owned | a		1.4
Operating Leases, Future Minimum Payments Due		$ 14,387
Scenario, Forecast [Member]
Principle Accounting Policies [Line Items]
Increase in accumulated other comprehensive loss due to change in comprehensive income reporting	$ 20,000
Pension and Other Postretirement Employee Benefit [Member]
Principle Accounting Policies [Line Items]
Net periodic pension and other postretirement employee benefit cost		13,200
Service cost		$ 6,800
Timber and Timberlands [Member] | Minimum [Member]
Principle Accounting Policies [Line Items]
Timber Volume Estimate, Review Period		5 years
Property, Plant and Equipment, Useful Life		5 years
Timber and Timberlands [Member] | Maximum [Member]
Principle Accounting Policies [Line Items]
Timber Volume Estimate, Review Period		10 years
Property, Plant and Equipment, Useful Life		20 years