Fair Values of Assets and Liabilities - Losses Recognized Related to Nonrecurring Fair Value Measurements of Individual Assets or Portfolios (Detail) - Fair Value, Measurements, Nonrecurring [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Loans [Member]
Fair Value Assets Measured On Nonrecurring Basis Losses Recognized [Line Items]
Losses recognized related to nonrecurring fair value measurements	$ 426	$ 122	$ 83
Other Assets [Member]
Fair Value Assets Measured On Nonrecurring Basis Losses Recognized [Line Items]
Losses recognized related to nonrecurring fair value measurements	$ 21	$ 17	$ 26