Summary of Significant Accounting Policies (Narrative) (Details) - Accounting Standards Update 2017-07 [Member] - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Nonoperating Income (Expense) [Member]
Organization And Basis Of Presentation [Line Items]
New accounting pronouncement, quantified affect	$ 156	$ 139
Operating Expense [Member]
Organization And Basis Of Presentation [Line Items]
New accounting pronouncement, quantified affect	$ 156	$ 139

Represents the quantification of the effect of adopting the new accounting standard or change in accounting principle expected by the entity to have a significant effect on the entity's financial statements.

No definition available.

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