Pension and Other Postretirement Benefits - Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension
Components of net periodic benefit cost:
Service cost	$ 63	$ 62	$ 70
Interest cost	72	92	92
Expected return on plan assets	(133)	(144)	(151)
Amortization of actuarial (gain) loss	57	44	60
Amortization of prior service cost	0	0	1
Total net periodic benefit cost	59	54	72
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	9	9	11
Interest cost	19	24	24
Expected return on plan assets	(23)	(23)	(25)
Amortization of actuarial (gain) loss	(1)	0	(1)
Amortization of prior service cost	0	(1)	1
Total net periodic benefit cost	$ 4	$ 9	$ 10