Retirement Benefits - Reconciliation of Level 3 Assets and Liabilities measured at Fair Value for Pension and Other Postretirement Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	$ 16,678	$ 18,573
Defined Benefit Plan, Plan Assets, Amount	18,590	16,678
Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	486	458
Relating to assets still held at the reporting date	59	(5)
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Sold	(7)	(1)
Defined Benefit Plan Purchases	67	54
Defined Benefit Plan Sales	(4)	(8)
Settlements	(102)	(12)
Transfers out of Level 3	(12)
Defined Benefit Plan, Plan Assets, Amount	487	486
Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | 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	216	232
Relating to assets still held at the reporting date	28	(14)
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Sold	(7)	(1)
Defined Benefit Plan Purchases	26	19
Defined Benefit Plan Sales	(4)	(8)
Settlements	(2)	(12)
Transfers out of Level 3	(12)
Defined Benefit Plan, Plan Assets, Amount	245	216
Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Security Individually Held [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	2	2
Relating to assets still held at the reporting date	3	0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Sold	0	0
Defined Benefit Plan Purchases	0	0
Defined Benefit Plan Sales	0	0
Settlements	0	0
Transfers out of Level 3	0
Defined Benefit Plan, Plan Assets, Amount	5	2
Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | 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	268	224
Relating to assets still held at the reporting date	28	9
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Sold	0	0
Defined Benefit Plan Purchases	41	35
Defined Benefit Plan Sales	0	0
Settlements	(100)	0
Transfers out of Level 3	0
Defined Benefit Plan, Plan Assets, Amount	237	268
Fair Value, Recurring [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	19,130
Defined Benefit Plan, Plan Assets, Amount	21,251	19,130
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	486
Defined Benefit Plan, Plan Assets, Amount	487	486
Fair Value, Recurring [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	16,720
Defined Benefit Plan, Plan Assets, Amount	18,710	16,720
Fair Value, Recurring [Member] | Pension Plan, Defined Benefit [Member] | 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	6,102
Defined Benefit Plan, Plan Assets, Amount	7,529	6,102
Fair Value, Recurring [Member] | Pension Plan, Defined Benefit [Member] | 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	1,066
Defined Benefit Plan, Plan Assets, Amount	1,166	1,066
Fair Value, Recurring [Member] | Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	486
Defined Benefit Plan, Plan Assets, Amount	487	486
Fair Value, Recurring [Member] | Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | 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	216
Defined Benefit Plan, Plan Assets, Amount	245	216
Fair Value, Recurring [Member] | Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | 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	268
Defined Benefit Plan, Plan Assets, Amount	$ 237	$ 268