Derivatives and Hedging Activities - Cumulative Basis Adjustments on Fair Value Hedges (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Certificate of deposit ($100,000 or more)	$ 100,000	$ 100,000
Securities available for sale	27,556,000,000	21,843,000,000
Long-term Debt [Member] | Interest Rate [Member]
Derivatives, Fair Value [Line Items]
Carrying amount of hedged item	8,182,000,000	8,408,000,000
Hedge accounting basis adjustment	416,000,000	240,000,000
Available-for-sale Securities [Member] | Interest Rate [Member]
Derivatives, Fair Value [Line Items]
Carrying amount of hedged item	2,080,000,000
Hedge accounting basis adjustment	(21,000,000)
Securities available for sale	2,500,000,000
Not Designated as Hedging Instrument [Member] | Long-term Debt [Member] | Interest Rate [Member]
Derivatives, Fair Value [Line Items]
Hedge accounting basis adjustment	$ (8,000,000)	$ (9,000,000)