Derivatives and Hedging Activities - Cumulative Basis Adjustments on Fair Value Hedges (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivatives, Fair Value [Line Items]
Fair Value	$ 40,594	$ 27,556
Long-term debt | Interest rate
Derivatives, Fair Value [Line Items]
Carrying amount of hedged item	8,077	8,182
Hedge accounting basis adjustment	205	416
Securities available for sale | Interest rate
Derivatives, Fair Value [Line Items]
Carrying amount of hedged item	6,280	2,080
Hedge accounting basis adjustment	168	21
Fair Value	7,700	2,500
Derivatives Not Designated as Hedging Instruments | Long-term debt | Interest rate
Derivatives, Fair Value [Line Items]
Hedge accounting basis adjustment	$ (7)	$ (8)