Fair Value Disclosures - Notional or Contractual Amounts and Fair Values of Off-Balance Sheet Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Commitments to extend, originate or purchase credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Notional/Contract Amount	$ 67,860	$ 64,395
Fair Value	259	250
Residential mortgage loans sold with recourse
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Notional/Contract Amount	490	578
Fair Value	5	7
Other loans sold with recourse
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Notional/Contract Amount	4,153	4,240
Fair Value	5	7
Letters of credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Notional/Contract Amount	2,466	2,786
Fair Value	$ 21	$ 27