Fair Value Hierarchy - Fair Value of Loans Held for Investment by Level (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	$ 6,448	$ 7,655
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	0	0
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	5,476	5,818
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	972	1,837
Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	871	996
Corporate | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	0	0
Corporate | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	440	359
Corporate | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	431	637
Commercial real estate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	551	1,146
Commercial real estate | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	0	0
Commercial real estate | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	281	435
Commercial real estate | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	270	711
Residential real estate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	4,028	4,511
Residential real estate | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	0	0
Residential real estate | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	3,977	4,437
Residential real estate | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	51	74
Other collateralized
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	850	716
Other collateralized | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	0	0
Other collateralized | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	708	576
Other collateralized | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	142	140
Other
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	148	286
Other | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	0	0
Other | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	70	11
Other | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total loans, gross, Fair Value	$ 78	$ 275