Long-Term Debt - Summary of Long-term Debt (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt	$ 41,297	$ 40,167
Parent Company [Member]
Debt Instrument [Line Items]
Long-term debt	20,924	18,602
Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term debt	20,373	21,565
Subsidiaries [Member] \| Other Consolidated Community Development and Tax-Advantaged Investment Vies, Capitalized Lease Obligations, Debt Issuance Fees, and Unrealized Gains and Losses and Deferred Amounts Relating to Derivative Instruments [Member]
Debt Instrument [Line Items]
Long-term debt	$ 1,110	848
Fixed Rate [Member] \| Parent Company [Member] \| Subordinated Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.95%
Debt Instrument Maturity Year, End	Dec. 31, 2022
Long-term debt	$ 1,300	1,300
Fixed Rate [Member] \| Parent Company [Member] \| Subordinated Debt One [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.60%
Debt Instrument Maturity Year, End	Dec. 31, 2024
Long-term debt	$ 1,000	1,000
Fixed Rate [Member] \| Parent Company [Member] \| Subordinated Notes Two [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.50%
Debt Instrument Maturity Year, End	Dec. 31, 2026
Long-term debt	$ 199	199
Fixed Rate [Member] \| Parent Company [Member] \| Subordinated Notes Three [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.10%
Debt Instrument Maturity Year, End	Dec. 31, 2026
Long-term debt	$ 1,000	1,000
Fixed Rate [Member] \| Parent Company [Member] \| Medium-term Notes [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity Year, start	Jan. 01, 2021
Debt Instrument Maturity Year, End	Dec. 31, 2030
Long-term debt	$ 15,492	13,820
Fixed Rate [Member] \| Parent Company [Member] \| Medium-term Notes [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	0.85%
Fixed Rate [Member] \| Parent Company [Member] \| Medium-term Notes [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.125%
Fixed Rate [Member] \| Parent Company [Member] \| Other Debt Issuance Fees and Unrealized Gains and Losses and Deferred Amounts Relating to Derivative Instruments [Member]
Debt Instrument [Line Items]
Long-term debt	$ 683	33
Fixed Rate [Member] \| Parent Company [Member] \| Subordinated Debt Four [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.00%
Debt Instrument Maturity Year, End	Dec. 31, 2029
Long-term debt	$ 1,000	1,000
Fixed Rate [Member] \| Subsidiaries [Member] \| Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity Year, start	Jan. 01, 2021
Debt Instrument Maturity Year, End	Dec. 31, 2026
Long-term debt	$ 1,003	1,106
Fixed Rate [Member] \| Subsidiaries [Member] \| Federal Home Loan Bank Advances [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	1.25%
Fixed Rate [Member] \| Subsidiaries [Member] \| Federal Home Loan Bank Advances [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	8.25%
Fixed Rate [Member] \| Subsidiaries [Member] \| Bank Notes [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity Year, start	Jan. 01, 2021
Debt Instrument Maturity Year, End	Dec. 31, 2025
Long-term debt	$ 9,100	9,550
Fixed Rate [Member] \| Subsidiaries [Member] \| Bank Notes [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	1.80%
Fixed Rate [Member] \| Subsidiaries [Member] \| Bank Notes [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.45%
Floating Rate [Member] \| Parent Company [Member] \| Medium-term Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	0.855%
Debt Instrument Maturity Year, End	Dec. 31, 2022
Long-term debt	$ 250	250
Floating Rate [Member] \| Subsidiaries [Member] \| Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity Year, start	Jan. 01, 2022
Debt Instrument Maturity Year, End	Dec. 31, 2026
Long-term debt	$ 3,272	3,272
Floating Rate [Member] \| Subsidiaries [Member] \| Federal Home Loan Bank Advances [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	0.474%
Floating Rate [Member] \| Subsidiaries [Member] \| Federal Home Loan Bank Advances [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	0.765%
Floating Rate [Member] \| Subsidiaries [Member] \| Bank Notes [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity Year, start	Jan. 01, 2021
Debt Instrument Maturity Year, End	Dec. 31, 2059
Long-term debt	$ 5,888	$ 6,789
Floating Rate [Member] \| Subsidiaries [Member] \| Bank Notes [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	0.653%

Contractual interest rate for funds borrowed, under the debt agreement.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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Latest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

+ References

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-Publisher FASB
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Earliest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Topic 470
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