Restructuring and Transaction Costs (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restructuring Cost And Reserve [Line Items]
Restructuring and transaction costs	$ 49	$ 84	$ 97
Current liabilities associated with discontinued operations		459
Long-term liabilities associated with discontinued operations		185
Reduction of workforce [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and transaction costs	41
Expense associated with accelerated awards	11
Reduction of workforce [Member]
Restructuring Cost And Reserve [Line Items]
Expense associated with accelerated awards		31	31
Reduction of workforce [Member] | Defined Benefit Settlements [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and transaction costs	9	7	14
Reduction of workforce [Member] | Employee Related Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and transaction costs		$ 84	$ 97
Prior years' restructurings [Member]
Restructuring Cost And Reserve [Line Items]
Current liabilities associated with discontinued operations	30
Long-term liabilities associated with discontinued operations	136
Underwriting, bank, legal and accounting fees	$ 8