Trading Cash Instruments - Fair Value, Cash Instruments, Measurement Inputs, Disclosure (Detail) $ in Millions	Mar. 31, 2020 USD ($) yr	Dec. 31, 2019 USD ($) yr
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total financial assets at fair value | $	$ 614,112	$ 539,568
Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total financial assets at fair value | $	30,776	23,068
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 | $	$ 130	$ 191
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	0.043	0.027
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	0.070	0.114
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	1.0	0.3
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	0.220	0.217
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	0.880	0.811
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	6.0	6.6
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	0.155	0.135
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	0.599	0.556
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	3.3	2.8
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 | $	$ 251	$ 231
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	0.014	0.012
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	0.040	0.054
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	2.1	2.3
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	0.092	0.120
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	0.330	0.304
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	14.3	12.4
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	0.044	0.058
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	0.160	0.163
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	8.3	5.7
Level 3 [Member] | Corporate debt instruments [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total financial assets at fair value | $	$ 719	$ 692
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	0.023	0.001
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	1.3	1.7
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	0.468	0.204
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	0.700	0.697
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	5.6	16.6
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	0.156	0.072
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	0.558	0.549
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	3.4	5.1