Pension and Other Postretirement Benefits - Narrative (Details) $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($)
Pension
Defined Benefit Plan Disclosure [Line Items]
Expected current year company contributions	$ 26
Contributions	15
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected current year company contributions	5
Contributions	$ 3

Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration