Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) issuer shares	Jan. 01, 2018 USD ($)	Dec. 31, 2016 USD ($) shares
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Period restricted investments are to be allocated to 401(k) plan participants	4 years
Allowance for doubtful accounts	$ 0		$ 0
Timber holdings valuation sample	20.00%
Cash, uninsured amount	$ 18,600,000
Debt securities, fair value	$ 76,245,000
Common stock equivalents (in shares) | shares	0		0
Reduction in retained earnings	$ (577,557,000)		$ (669,293,000)
U.S. Treasury securities
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Debt securities, fair value	9,900,000
Corporate debt securities
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Debt securities, fair value	$ 66,400,000
Number of issuers | issuer	6
Commercial paper
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Debt securities, fair value	$ 160,000,000
Number of issuers | issuer	12
External Credit Rating, Non Investment Grade | Preferred stock
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of issuers | issuer	5
Other marketable securities, noncurrent	$ 35,000,000
Retained Earnings | Subsequent Event | ASU 2014-09
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Effect of new accounting principal in period of adoption		$ 1,600,000
Retained Earnings | Subsequent Event | ASU 2016-01
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Reduction in retained earnings		$ 900,000