Benefit Plans (Narrative) (Details) $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($)
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional employer contributions	$ 19
Pension [Member] \| Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Anticipated additional employer contributions	25
Other Postretirement Benefits Plan [Member] \| Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Anticipated additional employer contributions	$ 70

Amount of contributions expected to be received by defined benefit plan from employer in fiscal year. Excludes contributions paid by employer in current fiscal year.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e2709-114920

Name:	us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsRemainderOfFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant