Benefit Plans - Summary of the Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net periodic pension cost:
Service cost	$ 59	$ 47	$ 179	$ 152
Interest cost	50	48	150	144
Estimated return on plan assets	(112)	(93)	(336)	(278)
Amortization and other	21	18	60	57
Net periodic benefit cost	$ 18	$ 20	$ 53	$ 75
Income statement location of interest cost			us-gaap:OtherNoninterestExpense
Income statement location of estimated return on plan assets			us-gaap:OtherNoninterestExpense
Income statement location of amortization and other			us-gaap:OtherNoninterestExpense