Supplemental Financial Information (Variable Interest Entities) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Variable Interest Entity [Line Items]
Total assets	$ 54,773		$ 52,663
Revenues	7,412	$ 6,499
Operating income (loss)	1,528	902
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Total assets	1,435		1,385
Total liabilities	233		$ 197
Revenues	71	18
Operating income (loss)	$ 5	$ (2)