Securities Securities - Credit Risk Profile of Held-to-maturity Securities (Details) $ in Millions	Mar. 31, 2020 USD ($)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	$ 12,983
Investment Grade [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	12,983
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	2,850
US Government-sponsored enterprises [Member] | Investment Grade [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	2,850
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	10,121
US Government agency issued or guaranteed [Member] | Investment Grade [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	10,121
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 subdivision [Member]
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Securities held-to-maturity	10
Obligations of U.S. states and political subdivision [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 subdivision [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