Fair Value Disclosures - Off-Balance Sheet Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Commitments to extend, originate or purchase credit
Derivative Instruments, Gain (Loss) [Line Items]
Notional/Contract Amount	$ 70,601	$ 67,860
Fair Value	312	259
Residential mortgage loans sold with recourse
Derivative Instruments, Gain (Loss) [Line Items]
Notional/Contract Amount	452	490
Fair Value	5	5
Other loans sold with recourse
Derivative Instruments, Gain (Loss) [Line Items]
Notional/Contract Amount	4,475	4,153
Fair Value	5	5
Letters of credit
Derivative Instruments, Gain (Loss) [Line Items]
Notional/Contract Amount	2,465	2,466
Fair Value	$ 20	$ 21