Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Settlement loss	$ 9	$ 5	$ 7
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	69
Contributions made to 401k retirement benefit plans	$ 201	211	142
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%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated net actuarial loss that will be reclassified from accumulated other comprehensive income into net periodic benefit costs over next year	$ 22
Estimated prior service cost that will be reclassified from accumulated other comprehensive income into net periodic benefit costs over next year	0
Accumulated benefit obligation for the defined benefit pension plans	1,878	1,742
Settlement loss	9	5	7
Tax deductible discretionary contributions	4	$ 4	$ 4
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Projected benefit obligation	104
Accumulated benefit obligation	102
Fair value of plan assets	39
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated net actuarial loss that will be reclassified from accumulated other comprehensive income into net periodic benefit costs over next year	0
Estimated prior service cost that will be reclassified from accumulated other comprehensive income into net periodic benefit costs over next year	7
Impact on postretirement benefit obligation of one percentage point increase in cost trend	23
Impact on service and interest costs of one percentage point increase in cost trend	1
Impact on postretirement benefit obligation of one percentage point decrease in cost trend	20
Impact on service and interest costs of one percentage point decrease in cost trend	$ 1
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	2028
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	6.00%
Ultimate health care cost trend rate	4.50%
Year in which ultimate trend rate reached	2028