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Debt (Debt Exchange) (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended		4 Months Ended	9 Months Ended	11 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2020	Sep. 15, 2020	Nov. 19, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Conversion [Line Items]
Date of exchange						Sep. 16, 2020	Nov. 20, 2019
Original debt				$ 1,047	$ 1,839
New debt		$ 1,842	$ 1,047			$ 250	$ 250	$ 250
New rate		3.839%	2.973%
Cash consideration		$ 373	$ 319
Accrued and unpaid interest		$ 19	$ 4			$ 4	$ 19
New due date		Mar. 20, 2060	Sep. 16, 2062
Debt exchange fees	$ 9	$ 15
Minimum [Member]
Debt Conversion [Line Items]
Original due date				May 01, 2037	Jun. 01, 2033
Maximum [Member]
Debt Conversion [Line Items]
Original due date				Mar. 01, 2049	Sep. 10, 2058

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The value of the financial instrument(s) that the original debt is being converted into in a noncash (or part noncash) transaction. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

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Expiration, mandatory redemption, or due date, in CCYY-MM-DD format, of the financial instrument issued in exchange for the original debt being converted in a noncash or part noncash transaction.

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