Financial Instruments (Narrative) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Senior Notes	$ 100,000
6% Senior notes [Member]
Debt Instrument [Line Items]
Senior Notes	495,452	$ 495,308
Carrying Value [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	495,452	495,308
Carrying Value [Member] \| 6% Senior notes [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure		495,308
Fair Value [Member] \| Fair value inputs, Level 2 [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	511,740	525,120
Fair Value [Member] \| 6% Senior notes [Member] \| Fair value inputs, Level 2 [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	$ 511,740	$ 525,120

Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16(a))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Name:	us-gaap_SeniorNotes
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant