Recent Accounting Pronouncements (Narrative) (Details) - USD ($)			3 Months Ended		9 Months Ended
		Jan. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in income before taxes			$ 65,691,000	$ 36,457,000	$ 144,158,000	$ 88,821,000
Restricted cash included in other long-term assets	[1]		$ 3,708,000		$ 3,708,000
Restricted Cash and Cash Equivalents, Noncurrent, Asset, Statement of Financial Position [Extensible List]			us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent
Operating lease description					we have operating leases covering office space, equipment and vehicles expiring at various dates through 2033.
Operating leases undiscounted cash flows							$ 14,400,000
Accounting Standards Update (ASU) No. 2014-09 [Member] | Effect of Change [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Adjustment to accumulated deficit upon adoption of new accounting standards			$ 0		$ 0
ASU No. 2017-07 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in income before taxes				$ 0		$ 0
ASU No. 2016-16 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative-effect adjustment to accumulated deficit		$ 1,300,000
ASU No. 2016-18 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Restricted cash included in other long-term assets			$ 3,700,000		$ 3,700,000
Restricted Cash and Cash Equivalents, Noncurrent, Asset, Statement of Financial Position [Extensible List]			us-gaap:OtherAssetsNoncurrent		us-gaap:OtherAssetsNoncurrent
ASU No. 2018-2 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Increase in accumulated other comprehensive loss due to change in comprehensive income reporting					$ 23,300,000

[1] Amounts included in restricted cash represent proceeds held by a qualified intermediary that are intended to be reinvested in timber and timberlands.