Fair Value of Assets and Liabilities - Nonrecurring Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	$ 301	$ 405
Loans and leases	55,435	58,232
Non Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans and leases	86	77
OREO	6	15
Other assets	10	9
Non Recurring | First Mortgages
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	1	1
Non Recurring | Small Business Administration And USDA
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	694	509
Non Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans and leases	0	0
OREO	0	0
Other assets	0	0
Non Recurring | Level 1 | First Mortgages
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	0	0
Non Recurring | Level 1 | Small Business Administration And USDA
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	0	0
Non Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans and leases	0	0
OREO	0	0
Other assets	0	0
Non Recurring | Level 2 | First Mortgages
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	0	0
Non Recurring | Level 2 | Small Business Administration And USDA
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	693	508
Non Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans and leases	86	77
OREO	6	15
Other assets	10	9
Non Recurring | Level 3 | First Mortgages
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	1	1
Non Recurring | Level 3 | Small Business Administration And USDA
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	$ 1	$ 1