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Summary of Significant Accounting Policies Equity Method Investments (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Equity Method Investments and Joint Ventures [Abstract]          
Equity Method Investments $ 35   $ 35   $ 38
Gain (Loss) on Sale of Previously Unissued Stock by Subsidiary or Equity Investee, Nonoperating Income $ 0 $ 15 $ 0 $ 15