Income Taxes (Unrecognized Tax Benefit Changes) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2020
Significant Change In Unrecognized Tax Benefits Is Reasonably Possible [Line Items]
Effective Income Tax Rate Reconciliation, Percent	45.10%	23.20%		(31.50%)	13.40%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Impairment Losses, Percent				27.60%
Effective income tax reconciliation, update to the forecasted AETR impact	15.90%
Effective Income Tax Rate Reconciliation, Change in State and Foreign Tax Rates, Percent	17.40%
Effective Income Tax Rate Reconciliation, Deferred Tax Assets, Percent				16.30%
Effective Income Tax Rate Reconciliation, Disposition of Business, Percent	61.10%			9.50%
Effective Income Tax Rate Reconciliation, Tax Settlement, Percent				8.30%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	5.90%			0.20%
Unrecognized Tax Benefits, Period Decrease	$ 440
Unrecognized Tax Benefits, Increase Resulting from Acquisition	240
Unrecognized Tax Benefits, Interest on Income Taxes Expense	12			$ 11
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	138			138
Forecast [Member]
Significant Change In Unrecognized Tax Benefits Is Reasonably Possible [Line Items]
Effective Income Tax Rate Reconciliation, Tax Settlement, Amount						$ 25
Minimum [Member]
Significant Change In Unrecognized Tax Benefits Is Reasonably Possible [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible, Estimated Range of Change	200			200
Minimum [Member] | Forecast [Member]
Significant Change In Unrecognized Tax Benefits Is Reasonably Possible [Line Items]
Effective Income Tax Rate Reconciliation, Tax Settlement, Amount			$ 75
Maximum [Member]
Significant Change In Unrecognized Tax Benefits Is Reasonably Possible [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible, Estimated Range of Change	$ 300			$ 300
Maximum [Member] | Forecast [Member]
Significant Change In Unrecognized Tax Benefits Is Reasonably Possible [Line Items]
Effective Income Tax Rate Reconciliation, Tax Settlement, Amount			$ 125