Fair Value Measurements - Fair Values of Assets and Liabilities (Significant Unobservable Level 3 Inputs Nonrecurring Basis) (Details) - Nonrecurring - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	$ 864	$ 260
OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	20	13
Bank premises and equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	26	27
Operating lease equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	35	6
Private equity investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	96	13
Commercial | Commercial loans held for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	24
Commercial | Commercial and industrial loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	422	169
Commercial | Commercial mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	78	12
Commercial | Commercial leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	4	20
Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	159
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	829	249
Level 3 | OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	20	13
Level 3 | Bank premises and equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	26	27
Level 3 | Operating lease equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	35	6
Level 3 | Private equity investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	69	2
Level 3 | Commercial | Commercial loans held for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	16
Level 3 | Commercial | Commercial and industrial loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	422	169
Level 3 | Commercial | Commercial mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	78	12
Level 3 | Commercial | Commercial leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	4	20
Level 3 | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	159
Level 3 | Comparable Company Analysis | Private equity investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	69	2
Level 3 | Comparable Company Analysis | Commercial | Commercial loans held for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	16
Level 3 | Appraised Value | OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	20	13
Level 3 | Appraised Value | Bank premises and equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	26	27
Level 3 | Appraised Value | Operating lease equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	35	6
Level 3 | Appraised Value | Commercial | Commercial and industrial loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	422	169
Level 3 | Appraised Value | Commercial | Commercial mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	78	12
Level 3 | Appraised Value | Commercial | Commercial leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	4	$ 20
Level 3 | Appraised Value | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	$ 159