Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Settlement loss	$ 29	$ 9	$ 5
Net amount of assets (liabilities) of cash, investment income receivable and amounts due to/from brokers, excluded from fair values of pension benefit assets and other benefit assets	64	64
Contributions made to 401k retirement benefit plans	$ 221	201	211
Increase in contributions following the enactment of the Tax Cuts and Jobs Act			58
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted-average target allocation for plan assets	44.00%
Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted-average target allocation for plan assets	48.00%
All Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted-average target allocation for plan assets	8.00%
Partnership investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Unfunded Commitment	$ 3	29
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation for the defined benefit pension plans	2,007	1,878
Settlement loss	29	9	5
Tax deductible discretionary contributions	7	$ 4	$ 4
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Projected benefit obligation	111
Accumulated benefit obligation	42
Defined Benefit Plan, Pension Plan with Project Benefit Obligation in Excess of Plan Assets [Abstract]
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation	113
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets	$ 42
Other Benefits [Member] | Pre-Medicare [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend rate to be used for next year to measure expected cost of other benefits	7.00%
Ultimate health care cost trend rate	4.50%
Year in which ultimate trend rate reached	2033
Other Benefits [Member] | Post-Medicare [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend rate to be used for next year to measure expected cost of other benefits	5.50%
Ultimate health care cost trend rate	4.50%
Year in which ultimate trend rate reached	2033