Fair Value Measurements (Details) - 6.50% Notes due 2024 - Fair Value, Inputs, Level 1 - Senior Notes - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Liabilities:
Long term debt, Carrying Amount	$ 900,000	$ 900,000
Long term debt, Fair Value	$ 798,750	$ 904,500

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant