Retirement Benefits - Reconciliation of Level 3 Assets and Liabilities measured at Fair Value for Pension and Other Postretirement Benefit Plans (Details) - Pension and other post retirement benefit [Member] - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	$ 487	$ 486
Relating to assets still held at the reporting date	31	59
Relating to assets sold during the period		(7)
Defined Benefit Plan Purchases	58	67
Defined Benefit Plan Sales		(4)
Settlements	(45)	(102)
Transfers into (out of) Level 3	299	(12)
Defined Benefit Plan, Plan Assets, Amount	830	487
Debt Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	245	216
Relating to assets still held at the reporting date	19	28
Relating to assets sold during the period		(7)
Defined Benefit Plan Purchases	34	26
Defined Benefit Plan Sales		(4)
Settlements	(3)	(2)
Transfers into (out of) Level 3	299	(12)
Defined Benefit Plan, Plan Assets, Amount	594	245
Private Placement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	237	268
Relating to assets still held at the reporting date	15	28
Relating to assets sold during the period		0
Defined Benefit Plan Purchases	24	41
Defined Benefit Plan Sales		0
Settlements	(42)	(100)
Transfers into (out of) Level 3	0	0
Defined Benefit Plan, Plan Assets, Amount	234	237
Equity Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	5	2
Relating to assets still held at the reporting date	(3)	3
Relating to assets sold during the period		0
Defined Benefit Plan Purchases	0	0
Defined Benefit Plan Sales		0
Settlements	0	0
Transfers into (out of) Level 3	0	0
Defined Benefit Plan, Plan Assets, Amount	$ 2	$ 5