Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended					9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Curtailments				$ 9
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6		$ 9			$ 19	$ 37
Interest cost	59		54			177	164
Expected return on plan assets	(71)		(76)			(214)	(230)
Recognized net actuarial loss	35		42			104	126
Amortization of prior service cost (benefit)	0		1			1	3
Settlements	4		4			8	4
Curtailments	0		0		$ (5)	0	14
Net periodic benefit cost	33		34			95	118
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1		2			5	6
Interest cost	7		7			21	21
Recognized net actuarial loss	1		1			3	5
Amortization of prior service cost (benefit)	(1)		(2)			(4)	(6)
Curtailments	0	$ 58	(28)			(58)	(28)
Net periodic benefit cost	$ 8		$ (20)			$ (33)	$ (2)