Fair Value Measurements (Narrative) (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Fair Value Disclosures [Abstract]
Sensitivity analysis of fair value, increase in liabilities, impact of 1 percent increase in discount rate	$ 6.6
Credit-risk-related contingent features	we have no credit-risk-related contingent features in our agreements with the financial institutions that would require us to post collateral for derivative positions in a liability position

Description of the existence and nature of credit-risk-related contingent features related to derivative instruments (and nonderivative instruments that are designated and qualify as hedging instruments) that are in a net liability position at the end of the reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624258-113959

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration