Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Amortization of:
Net periodic benefit cost	$ (5)	$ 10
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	89	101
Interest cost	221	201
Expected return on assets	(307)	(312)
Amortization of:
Prior service cost (benefit)	0	0
Actuarial loss	104	157
Net periodic benefit cost	107	147
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	24	28
Interest cost	47	43
Expected return on assets	(38)	(43)
Amortization of:
Prior service cost (benefit)	(45)	(46)
Actuarial loss	11	16
Net periodic benefit cost	$ (1)	$ (2)