Securities - Debt Securities, Held-to-maturity, Credit Quality Indicator (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 46,096	$ 34,483
Unrealized gain (loss)	$ 1,362
Ratings (percent)	0.00%
Agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 37,086	27,357
Unrealized gain (loss)	$ 1,107
Ratings (percent)	0.00%
U.S. Treasury
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 3,288	3,818
Unrealized gain (loss)	$ 103
Ratings (percent)	0.00%
U.S. government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 2,266	1,023
Unrealized gain (loss)	$ 1
Ratings (percent)	0.00%
Agency commercial mortgage-backed securities (“MBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 2,041	1,326
Unrealized gain (loss)	$ 107
Ratings (percent)	0.00%
Commercial paper/certificates of deposit (“CDs”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 295
Unrealized gain (loss)	$ 0
Ratings (percent)	0.00%
Sovereign debt/sovereign guaranteed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 983	756
Unrealized gain (loss)	$ 41
Ratings (percent)	0.00%
Non-agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 70	80
Unrealized gain (loss)	$ 2
Ratings (percent)	1.00%
Supranational
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 52	27
Unrealized gain (loss)	$ 1
Ratings (percent)	0.00%
State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 15	$ 17
Unrealized gain (loss)	$ 0
Ratings (percent)	86.00%
AAA/ AA-
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	99.00%
AAA/ AA- | Agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	100.00%
AAA/ AA- | U.S. Treasury
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	100.00%
AAA/ AA- | U.S. government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	100.00%
AAA/ AA- | Agency commercial mortgage-backed securities (“MBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	100.00%
AAA/ AA- | Commercial paper/certificates of deposit (“CDs”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
AAA/ AA- | Sovereign debt/sovereign guaranteed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	100.00%
AAA/ AA- | Non-agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	39.00%
AAA/ AA- | Supranational
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	100.00%
AAA/ AA- | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	6.00%
A/ A-
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A/ A- | Agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A/ A- | U.S. Treasury
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A/ A- | U.S. government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A/ A- | Agency commercial mortgage-backed securities (“MBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A/ A- | Commercial paper/certificates of deposit (“CDs”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A/ A- | Sovereign debt/sovereign guaranteed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A/ A- | Non-agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	46.00%
A/ A- | Supranational
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A/ A- | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	2.00%
BBB/ BBB-
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BBB/ BBB- | Agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BBB/ BBB- | U.S. Treasury
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BBB/ BBB- | U.S. government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BBB/ BBB- | Agency commercial mortgage-backed securities (“MBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BBB/ BBB- | Commercial paper/certificates of deposit (“CDs”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BBB/ BBB- | Sovereign debt/sovereign guaranteed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BBB/ BBB- | Non-agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	2.00%
BBB/ BBB- | Supranational
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BBB/ BBB- | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	6.00%
BB and lower
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BB and lower | Agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BB and lower | U.S. Treasury
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BB and lower | U.S. government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BB and lower | Agency commercial mortgage-backed securities (“MBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BB and lower | Commercial paper/certificates of deposit (“CDs”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BB and lower | Sovereign debt/sovereign guaranteed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BB and lower | Non-agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	12.00%
BB and lower | Supranational
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
BB and lower | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A1+/A2/SP-1+
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	1.00%
A1+/A2/SP-1+ | Agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A1+/A2/SP-1+ | U.S. Treasury
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A1+/A2/SP-1+ | U.S. government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A1+/A2/SP-1+ | Agency commercial mortgage-backed securities (“MBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A1+/A2/SP-1+ | Commercial paper/certificates of deposit (“CDs”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	100.00%
A1+/A2/SP-1+ | Sovereign debt/sovereign guaranteed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A1+/A2/SP-1+ | Non-agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A1+/A2/SP-1+ | Supranational
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%
A1+/A2/SP-1+ | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%