Benefit Plans - Summary of the 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
Net periodic pension cost:
Service cost	$ 55	$ 60	$ 109	$ 120
Interest cost	54	50	111	100
Estimated return on plan assets	(114)	(112)	(227)	(224)
Amortization and other	26	19	51	39
Net periodic benefit cost	$ 21	$ 17	$ 44	$ 35