Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Curtailments		$ 9
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6	8	$ 13	$ 28
Interest cost	59	55	118	110
Expected return on plan assets	(71)	(77)	(143)	(154)
Recognized net actuarial loss	34	42	69	84
Amortization of prior service cost (benefit)	1	1	1	2
Settlements	2	0	4	0
Curtailments	0	9	0	14
Net periodic benefit cost	31	38	62	84
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	2	4	4
Interest cost	7	7	14	14
Recognized net actuarial loss	1	2	2	4
Amortization of prior service cost (benefit)	(2)	(2)	(3)	(4)
Curtailments	0	0	(58)	0
Net periodic benefit cost	$ 8	$ 9	$ (41)	$ 18