Securities - Credit Risk Profile of Held-to-maturity Securities (Details) $ in Millions	Sep. 30, 2020 USD ($)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	$ 10,226
Investment Grade [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	10,226
Non-Investment Grade [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	0
US Government-sponsored enterprises [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	2,195
US Government-sponsored enterprises [Member] | Investment Grade [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	2,195
US Government-sponsored enterprises [Member] | Non-Investment Grade [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	0
US Government agency issued or guaranteed [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	8,019
US Government agency issued or guaranteed [Member] | Investment Grade [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	8,019
US Government agency issued or guaranteed [Member] | Non-Investment Grade [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	0
Obligations of U.S. states and political subdivisions [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	10
Obligations of U.S. states and political subdivisions [Member] | Investment Grade [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	10
Obligations of U.S. states and political subdivisions [Member] | Non-Investment Grade [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	0
Asset-backed securities collateralized by residential mortgages [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	2
Asset-backed securities collateralized by residential mortgages [Member] | Investment Grade [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	2
Asset-backed securities collateralized by residential mortgages [Member] | Non-Investment Grade [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	$ 0