Benefit Plans - Summary of the Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net periodic pension cost:
Service cost	$ 54	$ 60
Interest cost	57	50
Estimated return on plan assets	(113)	(112)
Amortization and other	25	20
Net periodic benefit cost	$ 23	$ 18