Trading Cash Instruments - Fair Value, Cash Instruments, Measurement Inputs, Disclosure (Detail) $ in Millions	Sep. 30, 2020 USD ($) yr	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($) yr
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total financial assets at fair value | $	$ 630,012	$ 641,711	$ 539,568
Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total financial assets at fair value | $	27,677	$ 29,426	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 | $	$ 196		$ 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	0.020		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	0.197		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	0.3		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	0.938		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	9.2		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	0.099		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	0.614		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	5.3		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 | $	$ 180		$ 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	0.016		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	0.047		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	1.2		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	0.142		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	0.366		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	14.8		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	0.059		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	0.193		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	4.8		5.7
Level 3 [Member] | Corporate debt instruments [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total financial assets at fair value | $	$ 782		$ 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	0.012		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	1.9		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	0.325		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	0.699		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	6.1		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	0.119		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	0.590		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	3.3		5.1