Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Curtailments	$ 9
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	8	$ 21	$ 28	$ 44
Interest cost	55	58	110	116
Expected return on plan assets	(77)	(82)	(154)	(165)
Recognized net actuarial loss	42	55	84	110
Amortization of prior service cost (benefit)	1	2	2	3
Curtailments	9	0	14	0
Net periodic benefit cost	38	54	84	108
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	2	4	4
Interest cost	7	7	14	15
Recognized net actuarial loss	2	1	4	2
Amortization of prior service cost (benefit)	(2)	(2)	(4)	(4)
Net periodic benefit cost	$ 9	$ 8	$ 18	$ 17