Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 46	$ 57	$ 138	$ 171
Interest cost	56	54	170	162
Expected return on plan assets	(73)	(73)	(218)	(219)
Amortization of unrecognized (gain) loss	29	32	87	97
Amortization of prior service credit	0	0	0	0
Settlement loss	2	0	5	0
Total	60	70	182	211
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	3	7	9
Interest cost	10	16	39	46
Expected return on plan assets	0	(1)	(1)	(1)
Amortization of unrecognized (gain) loss	(12)	(8)	(42)	(24)
Amortization of prior service credit	(23)	(9)	(42)	(28)
Settlement loss	0	0		0
Total	$ (23)	$ 1	$ (39)	$ 2