Summary of Significant Accounting Policies - Prospective Adoption of New Accounting Pronouncements (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Item Effected [Line Items]
Other Net Periodic Benefit Credit	$ 215	$ 201	$ 233

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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