Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Ten Percent Change in Implied Volatility [member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Increase (Decrease) in Risk Management Assets and Liabilities	$ 2	$ 3

Amount of increase (decrease) in fair value of financial instruments other than those that are purchased and held principally for the purpose of hedging or mitigating market risk, credit risk, legal risk, operational risk and other types of risk.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Name:	us-gaap_IncreaseDecreaseInRiskManagementAssetsAndLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration