Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Curtailments		$ 9
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 9		$ 23	$ 37	$ 67
Interest cost	54		59	164	175
Expected return on plan assets	(76)		(83)	(230)	(248)
Recognized net actuarial loss	42		55	126	165
Amortization of prior service cost (benefit)	1		1	3	4
Settlements	(4)		0	(4)	0
Curtailments	0		0	14	0
Net periodic benefit cost	34		55	118	163
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2		2	6	6
Interest cost	7		7	21	22
Recognized net actuarial loss	1		2	5	4
Amortization of prior service cost (benefit)	(2)		(2)	(6)	(6)
Curtailments	(28)		0	(28)	0
Net periodic benefit cost	$ (20)		$ 9	$ (2)	$ 26