Summary of Significant Accounting Policies (Noncontrolling Interest) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Redeemable Noncontrolling Interest [Line Items]
Assets	$ 56,715	$ 52,580
Liabilities	44,063	38,695
Unnamed subsidiary
Redeemable Noncontrolling Interest [Line Items]
Assets		299
Liabilities		41
Net (loss) income	$ 14	$ 21	$ 9

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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