Pension and Other Postretirement Benefits - Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension
Components of net periodic benefit cost:
Service cost	$ 60	$ 63	$ 62
Interest cost	72	72	92
Expected return on plan assets	(126)	(133)	(144)
Amortization of actuarial (gain) loss	43	57	44
Amortization of prior service cost	0	0	0
Total net periodic benefit cost	49	59	54
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	8	9	9
Interest cost	18	19	24
Expected return on plan assets	(25)	(23)	(23)
Amortization of actuarial (gain) loss	(2)	(1)	0
Amortization of prior service cost	0	0	(1)
Total net periodic benefit cost	$ (1)	$ 4	$ 9