Pension and Other Postretirement Employee Benefit Plans - Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined benefit pension plans
Components of net periodic benefit cost
Benefits earned during the period	$ 89	$ 82	$ 179	$ 164
Interest cost on benefit obligations	139	150	278	299
Expected return on plan assets	(247)	(242)	(495)	(483)
Amortization:
Net (gain)/loss	26	62	52	124
Prior service cost/(credit)	(6)	(9)	(12)	(18)
Settlement	0	0	0	(3)
Net periodic defined benefit cost	1	43	2	83
Other defined benefit pension plans	9	6	15	10
Total defined benefit plans	10	49	17	93
Total defined contribution plans	222	210	432	396
Total pension and OPEB cost included in noninterest expense	232	259	449	489
OPEB plans
Components of net periodic benefit cost
Benefits earned during the period	0	0	0	0
Interest cost on benefit obligations	6	7	12	14
Expected return on plan assets	(26)	(24)	(52)	(48)
Amortization:
Net (gain)/loss	0	0	0	0
Prior service cost/(credit)	0	0	0	0
Settlement	0	0	0	0
Net periodic defined benefit cost	(20)	(17)	(40)	(34)
Total defined benefit plans	(20)	(17)	(40)	(34)
Total pension and OPEB cost included in noninterest expense	$ (20)	$ (17)	$ (40)	$ (34)