Fair Value Disclosures - Off-Balance Sheet Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Commitments to extend, originate or purchase credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Notional/Contract Amount	$ 73,918	$ 72,435
Fair Value	302	280
Residential mortgage loans sold with recourse
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Notional/Contract Amount	404	419
Fair Value	3	3
CRE mortgages serviced for others covered by recourse provisions
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Notional/Contract Amount	4,685	4,699
Fair Value	6	6
Letters of credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Notional/Contract Amount	2,267	2,389
Fair Value	$ 17	$ 18