Background and Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption, net of tax	$ 10,098	$ 10,117		$ 9,868
Operating lease assets	67	105	$ 126
Operating lease liability	104		$ 133
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption, net of tax	$ 48	67		(182)	$ (713)
Cumulative effect of new accounting principle in period of adoption, tax		(1)			(49)
Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption, net of tax		$ 3		$ 22	$ 141