Fair Value Disclosures - Off-Balance Sheet Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Commitments to extend, originate or purchase credit
Derivative Instruments, Gain (Loss) [Line Items]
Notional/Contract Amount	$ 69,814	$ 67,860
Fair Value	324	259
Residential mortgage loans sold with recourse
Derivative Instruments, Gain (Loss) [Line Items]
Notional/Contract Amount	473	490
Fair Value	5	5
Other loans sold with recourse
Derivative Instruments, Gain (Loss) [Line Items]
Notional/Contract Amount	4,175	4,153
Fair Value	5	5
Letters of credit
Derivative Instruments, Gain (Loss) [Line Items]
Notional/Contract Amount	2,460	2,466
Fair Value	$ 22	$ 21