Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Amortization of:
Net periodic benefit cost	$ 2	$ 12	$ (8)	$ 33
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	89	100	267	303
Interest cost	221	201	663	602
Expected return on assets	(306)	(312)	(918)	(939)
Amortization of:
Prior service cost (benefit)	0	0	0	1
Actuarial loss	104	158	310	472
Settlement charges	7	0	7	1
Contractual Termination Benefits	1	0	1	0
Net periodic benefit cost	116	147	330	440
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	23	28	70	84
Interest cost	47	43	141	131
Expected return on assets	(38)	(43)	(115)	(130)
Amortization of:
Prior service cost (benefit)	(45)	(47)	(134)	(140)
Actuarial loss	11	18	34	50
Settlement charges	0	0	0	0
Contractual Termination Benefits	0	0	0	0
Net periodic benefit cost	$ (2)	$ (1)	$ (4)	$ (5)