Fair Values of Assets and Liabilities - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2020		Dec. 31, 2019		Mar. 31, 2019	Dec. 31, 2018
Financial Assets
Cash and due from banks	$ 46,805	[1]	$ 22,405	[1]	$ 18,115	$ 21,453
Loans	312,089		292,082
Financial Liabilities
Long-term debt	52,298		40,167
Carrying Amount [Member]
Financial Assets
Cash and due from banks	46,805		22,405
Federal funds sold and securities purchased under resale agreements	236		1,036
Loans held for sale	43		45
Loans	312,089		292,082
Other	2,334		1,923
Financial Liabilities
Time deposits	42,472		42,894
Short-term borrowings	24,730		22,095
Long-term debt	52,298		40,167
Other	3,512		3,678
Fair Value [Member]
Financial Assets
Cash and due from banks	46,805		22,405
Federal funds sold and securities purchased under resale agreements	236		1,036
Loans held for sale	43		43
Loans	317,824		297,241
Other	2,334		1,923
Financial Liabilities
Time deposits	42,116		42,831
Short-term borrowings	24,471		21,961
Long-term debt	52,371		41,077
Other	3,512		3,678
Fair Value [Member] | Level 1 [Member]
Financial Assets
Cash and due from banks	46,805		22,405
Fair Value [Member] | Level 2 [Member]
Financial Assets
Federal funds sold and securities purchased under resale agreements	236		1,036
Other	1,459		929
Financial Liabilities
Time deposits	42,116		42,831
Short-term borrowings	24,471		21,961
Long-term debt	52,371		41,077
Other	1,316		1,342
Fair Value [Member] | Level 3 [Member]
Financial Assets
Loans held for sale	43		43
Loans	317,824		297,241
Other	875		994
Financial Liabilities
Other	$ 2,196		$ 2,336

[1] Includes only collateral pledged by the Company where counterparties have the right to sell or pledge the collateral.