Borrowings and Other Obligations Fair and carrying value of long-term debt (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt	$ 2,098	$ 2,098
Secured Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt	457	455
Fair Value, Inputs, Level 2 | Senior Notes | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	2,018	1,628
Fair Value, Inputs, Level 2 | Secured Debt | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	524	507
Fair Value, Inputs, Level 2 | Secured and Senior Notes | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	2,542	2,135
Fair Value, Inputs, Level 2 | Secured and Senior Notes | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	$ 2,555	$ 2,553