Borrowings and Other Obligations (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Aug. 04, 2020	Mar. 31, 2020	Dec. 12, 2019
Short-term Debt [Line Items]
Debt instrument, unused borrowing capacity, amount	$ 55
Short-term Debt	11	$ 13
Minimum Liquidity Covenant	175
Interest Payable	80	34
LC Credit Agreement Letters of Credit
Short-term Debt [Line Items]
Letters of credit outstanding, amount	160
Other short-term bank loans
Short-term Debt [Line Items]
Short-term Debt	0	$ 4
Committed Letters of Credit
Short-term Debt [Line Items]
Letters of credit outstanding, amount				$ 328
Letters of credit outstanding, cash collateral	163
Committed letters of credit
Short-term Debt [Line Items]
Letters of credit outstanding, amount	168
LC Credit Agreement
Short-term Debt [Line Items]
Credit agreement, maximum capacity					$ 195
Amended LC Credit Agreement [Member]
Short-term Debt [Line Items]
Credit agreement, maximum capacity	$ 215
Senior Notes | Senior Notes 8.75 Percent Due 2024
Short-term Debt [Line Items]
Stated interest rate on debt			8.75%
Face amount of debt			$ 500
Senior Notes | Exit Notes, 11.00 Percent Due 2024
Short-term Debt [Line Items]
Stated interest rate on debt					11.00%
Face amount of debt					$ 2,100