Benefit Plans - Summary of the Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net Periodic Pension Cost:
Service cost	$ 60	$ 53	$ 120	$ 105
Interest cost	50	47	100	96
Estimated return on plan assets	(112)	(92)	(224)	(185)
Amortization and other	19	19	39	39
Net periodic benefit cost	$ 17	$ 27	$ 35	$ 55
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