Trading Cash Instruments - Fair Value, Cash Instruments, Measurement Inputs, Disclosure (Detail) $ in Millions	Dec. 31, 2019 USD ($) yr	Dec. 31, 2018 USD ($) yr
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total financial assets at fair value | $	$ 539,568	$ 501,712
Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total financial assets at fair value | $	23,068	22,181
Level 3 [Member] | Loans and Securities Backed by Commercial Real Estate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total financial assets at fair value | $	$ 191	$ 332
Level 3 [Member] | Loans and Securities Backed by Commercial Real Estate [Member] | Minimum [Member] | Fair Value Unobservable Inputs, Yield [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.027	0.069
Level 3 [Member] | Loans and Securities Backed by Commercial Real Estate [Member] | Minimum [Member] | Fair Value Unobservable Inputs, Recovery Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.114	0.170
Level 3 [Member] | Loans and Securities Backed by Commercial Real Estate [Member] | Minimum [Member] | Fair Value Unobservable Inputs, Duration [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.3	0.4
Level 3 [Member] | Loans and Securities Backed by Commercial Real Estate [Member] | Maximum [Member] | Fair Value Unobservable Inputs, Yield [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.217	0.225
Level 3 [Member] | Loans and Securities Backed by Commercial Real Estate [Member] | Maximum [Member] | Fair Value Unobservable Inputs, Recovery Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.811	0.784
Level 3 [Member] | Loans and Securities Backed by Commercial Real Estate [Member] | Maximum [Member] | Fair Value Unobservable Inputs, Duration [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	6.6	7.1
Level 3 [Member] | Loans and Securities Backed by Commercial Real Estate [Member] | Weighted Average [Member] | Fair Value Unobservable Inputs, Yield [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.135	0.142
Level 3 [Member] | Loans and Securities Backed by Commercial Real Estate [Member] | Weighted Average [Member] | Fair Value Unobservable Inputs, Recovery Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.556	0.53
Level 3 [Member] | Loans and Securities Backed by Commercial Real Estate [Member] | Weighted Average [Member] | Fair Value Unobservable Inputs, Duration [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.028	0.033
Level 3 [Member] | Loans and Securities Backed by Residential Real Estate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total financial assets at fair value | $	$ 231	$ 348
Level 3 [Member] | Loans and Securities Backed by Residential Real Estate [Member] | Minimum [Member] | Fair Value Unobservable Inputs, Yield [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.012	0.028
Level 3 [Member] | Loans and Securities Backed by Residential Real Estate [Member] | Minimum [Member] | Fair Value Unobservable Inputs, Cumulative Loss Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.054	0.083
Level 3 [Member] | Loans and Securities Backed by Residential Real Estate [Member] | Minimum [Member] | Fair Value Unobservable Inputs, Duration [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	2.3	1.5
Level 3 [Member] | Loans and Securities Backed by Residential Real Estate [Member] | Maximum [Member] | Fair Value Unobservable Inputs, Yield [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.120	0.118
Level 3 [Member] | Loans and Securities Backed by Residential Real Estate [Member] | Maximum [Member] | Fair Value Unobservable Inputs, Cumulative Loss Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.304	0.377
Level 3 [Member] | Loans and Securities Backed by Residential Real Estate [Member] | Maximum [Member] | Fair Value Unobservable Inputs, Duration [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	12.4	14.0
Level 3 [Member] | Loans and Securities Backed by Residential Real Estate [Member] | Weighted Average [Member] | Fair Value Unobservable Inputs, Yield [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.058	0.059
Level 3 [Member] | Loans and Securities Backed by Residential Real Estate [Member] | Weighted Average [Member] | Fair Value Unobservable Inputs, Cumulative Loss Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.163	0.153
Level 3 [Member] | Loans and Securities Backed by Residential Real Estate [Member] | Weighted Average [Member] | Fair Value Unobservable Inputs, Duration [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.057	0.086
Level 3 [Member] | Corporate debt instruments [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total financial assets at fair value | $	$ 692	$ 912
Level 3 [Member] | Corporate debt instruments [Member] | Minimum [Member] | Fair Value Unobservable Inputs, Yield [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.001	0.007
Level 3 [Member] | Corporate debt instruments [Member] | Minimum [Member] | Fair Value Unobservable Inputs, Recovery Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.000	0.000
Level 3 [Member] | Corporate debt instruments [Member] | Minimum [Member] | Fair Value Unobservable Inputs, Duration [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	1.7	0.4
Level 3 [Member] | Corporate debt instruments [Member] | Maximum [Member] | Fair Value Unobservable Inputs, Yield [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.204	0.173
Level 3 [Member] | Corporate debt instruments [Member] | Maximum [Member] | Fair Value Unobservable Inputs, Recovery Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.697	0.750
Level 3 [Member] | Corporate debt instruments [Member] | Maximum [Member] | Fair Value Unobservable Inputs, Duration [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	16.6	13.5
Level 3 [Member] | Corporate debt instruments [Member] | Weighted Average [Member] | Fair Value Unobservable Inputs, Yield [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.072	0.084
Level 3 [Member] | Corporate debt instruments [Member] | Weighted Average [Member] | Fair Value Unobservable Inputs, Recovery Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.549	0.612
Level 3 [Member] | Corporate debt instruments [Member] | Weighted Average [Member] | Fair Value Unobservable Inputs, Duration [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Inputs, Asset	0.051	0.036