Retirement Benefits (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Contribution Plan [Abstract]
Defined contribution plan, employer contributions	$ 191	$ 104
Pension Benefits
Components of Net Periodic Benefit Cost
Service cost	92	99
Interest cost	340	290
Expected return on plan assets	(525)	(529)
Amortization of prior service credit	(15)	(15)
Net periodic benefit cost (benefit)	(108)	(155)
Defined benefit plan, contributions by Employer	23	22
OPB
Components of Net Periodic Benefit Cost
Service cost	4	5
Interest cost	20	19
Expected return on plan assets	(23)	(25)
Amortization of prior service credit	(1)	(5)
Net periodic benefit cost (benefit)	0	(6)
Defined benefit plan, contributions by Employer	$ 12	$ 11