Fair Value Disclosures - Off-Balance Sheet Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Commitments to extend, originate or purchase credit
Derivative Instruments, Gain (Loss) [Line Items]
Notional/Contract Amount	$ 72,059	$ 67,860
Fair Value	275	259
Residential mortgage loans sold with recourse
Derivative Instruments, Gain (Loss) [Line Items]
Notional/Contract Amount	436	490
Fair Value	4	5
Other loans sold with recourse
Derivative Instruments, Gain (Loss) [Line Items]
Notional/Contract Amount	4,635	4,153
Fair Value	6	5
Letters of credit
Derivative Instruments, Gain (Loss) [Line Items]
Notional/Contract Amount	2,406	2,466
Fair Value	$ 19	$ 21