Cash Instruments - Fair Value, Cash Instruments, Measurement Inputs, Disclosure (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total financial assets at fair value	$ 508,954	$ 497,693
Loans and Securities Backed by Commercial Real Estate [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Yield	4.50%	3.70%
Fair Value Unobservable Inputs, Recovery Rate	17.40%	8.90%
Fair Value Unobservable Inputs, Duration	8 months 12 days	9 months 18 days
Loans and Securities Backed by Commercial Real Estate [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Yield	22.00%	23.00%
Fair Value Unobservable Inputs, Recovery Rate	97.90%	99.00%
Fair Value Unobservable Inputs, Duration	6 years 1 month 6 days	6 years 2 months 12 days
Loans and Securities Backed by Commercial Real Estate [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Yield	12.30%	13.00%
Fair Value Unobservable Inputs, Recovery Rate	61.30%	60.60%
Fair Value Unobservable Inputs, Duration	1 year 10 months 24 days	2 years 1 month 6 days
Loans and Securities Backed by Residential Real Estate [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Yield	1.90%	0.80%
Fair Value Unobservable Inputs, Cumulative Loss Rate	9.40%	8.90%
Fair Value Unobservable Inputs, Duration	1 year 1 month 6 days	1 year 1 month 6 days
Loans and Securities Backed by Residential Real Estate [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Yield	15.00%	15.60%
Fair Value Unobservable Inputs, Cumulative Loss Rate	46.80%	47.10%
Fair Value Unobservable Inputs, Duration	14 years 7 months 6 days	16 years 1 month 6 days
Loans and Securities Backed by Residential Real Estate [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Yield	8.30%	8.70%
Fair Value Unobservable Inputs, Cumulative Loss Rate	25.70%	24.20%
Fair Value Unobservable Inputs, Duration	6 years 9 months 18 days	7 years 3 months 18 days
Corporate Loans And Debt Securities [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Yield	2.60%	2.50%
Fair Value Unobservable Inputs, Recovery Rate	0.00%	0.00%
Fair Value Unobservable Inputs, Duration	3 months 18 days	7 months 6 days
Corporate Loans And Debt Securities [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Yield	24.50%	25.00%
Fair Value Unobservable Inputs, Recovery Rate	94.00%	85.00%
Fair Value Unobservable Inputs, Duration	5 years 4 months 24 days	15 years 8 months 12 days
Corporate Loans And Debt Securities [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Yield	10.40%	10.30%
Fair Value Unobservable Inputs, Recovery Rate	56.50%	56.50%
Fair Value Unobservable Inputs, Duration	2 years 10 months 24 days	2 years 10 months 24 days
Equity Securities [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Multiples	0.8	0.8
Fair Value Unobservable Inputs, Discount Rate/Yield	6.50%	6.50%
Fair Value Unobservable Inputs, Capitalization Rates	4.10%	4.20%
Equity Securities [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Multiples	24.6	19.7
Fair Value Unobservable Inputs, Discount Rate/Yield	30.00%	25.00%
Fair Value Unobservable Inputs, Capitalization Rates	12.90%	12.50%
Equity Securities [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Multiples	7.0	6.8
Fair Value Unobservable Inputs, Discount Rate/Yield	16.20%	16.00%
Fair Value Unobservable Inputs, Capitalization Rates	6.60%	6.80%
Other Cash Instruments [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Yield	2.40%	1.90%
Fair Value Unobservable Inputs, Recovery Rate	2.70%	0.00%
Fair Value Unobservable Inputs, Duration	9 months 18 days	10 months 24 days
Other Cash Instruments [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Yield	16.10%	14.00%
Fair Value Unobservable Inputs, Recovery Rate	93.00%	93.00%
Fair Value Unobservable Inputs, Duration	12 years	12 years
Other Cash Instruments [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Unobservable Inputs, Yield	9.00%	8.80%
Fair Value Unobservable Inputs, Recovery Rate	82.60%	61.40%
Fair Value Unobservable Inputs, Duration	3 years 10 months 24 days	4 years 3 months 18 days
Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total financial assets at fair value	$ 23,288	$ 23,280
Level 3 [Member] | Loans and Securities Backed by Commercial Real Estate [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total financial assets at fair value	1,604	1,645
Level 3 [Member] | Loans and Securities Backed by Residential Real Estate [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total financial assets at fair value	830	845
Level 3 [Member] | Corporate Loans And Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total financial assets at fair value	4,553	4,640
Level 3 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total financial assets at fair value	10,715	10,263
Level 3 [Member] | Other Cash Instruments [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total financial assets at fair value	$ 622	$ 642