Securities - Debt Securities, Held-to-maturity, Credit Quality Indicator (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 56,267	$ 47,946
Unrealized gain (loss)	$ 441	$ 1,278
Ratings (percent)	0.00%	0.00%
Agency residential mortgage-backed securities (“RMBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 37,607	$ 38,355
Unrealized gain (loss)	$ 401	$ 1,041
Ratings (percent)	0.00%	0.00%
U.S. Treasury
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 10,528	$ 2,938
Unrealized gain (loss)	$ 21	$ 90
Ratings (percent)	0.00%	0.00%
U.S. government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 2,879	$ 2,816
Unrealized gain (loss)	$ (40)	$ (2)
Ratings (percent)	0.00%	0.00%
Agency commercial mortgage-backed securities (“MBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 4,170	$ 2,659
Unrealized gain (loss)	$ 35	$ 103
Ratings (percent)	0.00%	0.00%
Sovereign debt/sovereign guaranteed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 969	$ 1,050
Unrealized gain (loss)	$ 20	$ 42
Ratings (percent)	0.00%	0.00%
Non-agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 46	$ 58
Unrealized gain (loss)	$ 3	$ 3
Ratings (percent)	1.00%	1.00%
Supranational
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 54	$ 55
Unrealized gain (loss)	$ 0	$ 0
Ratings (percent)	0.00%	0.00%
State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost	$ 14	$ 15
Unrealized gain (loss)	$ 1	$ 1
Ratings (percent)	88.00%	86.00%
AAA/ AA-
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	100.00%	100.00%
AAA/ AA- | Agency residential mortgage-backed securities (“RMBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	100.00%	100.00%
AAA/ AA- | U.S. Treasury
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	100.00%	100.00%
AAA/ AA- | U.S. government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	100.00%	100.00%
AAA/ AA- | Agency commercial mortgage-backed securities (“MBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	100.00%	100.00%
AAA/ AA- | Sovereign debt/sovereign guaranteed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	100.00%	98.00%
AAA/ AA- | Non-agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	24.00%	28.00%
AAA/ AA- | Supranational
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	100.00%	100.00%
AAA/ AA- | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	5.00%	6.00%
A+/ A-
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
A+/ A- | Agency residential mortgage-backed securities (“RMBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
A+/ A- | U.S. Treasury
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
A+/ A- | U.S. government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
A+/ A- | Agency commercial mortgage-backed securities (“MBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
A+/ A- | Sovereign debt/sovereign guaranteed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
A+/ A- | Non-agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	59.00%	55.00%
A+/ A- | Supranational
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
A+/ A- | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	2.00%	2.00%
BBB+/ BBB-
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
BBB+/ BBB- | Agency residential mortgage-backed securities (“RMBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
BBB+/ BBB- | U.S. Treasury
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
BBB+/ BBB- | U.S. government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
BBB+/ BBB- | Agency commercial mortgage-backed securities (“MBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
BBB+/ BBB- | Sovereign debt/sovereign guaranteed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
BBB+/ BBB- | Non-agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	1.00%	2.00%
BBB+/ BBB- | Supranational
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
BBB+/ BBB- | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	5.00%	6.00%
BB+ and lower
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
BB+ and lower | Agency residential mortgage-backed securities (“RMBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
BB+ and lower | U.S. Treasury
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
BB+ and lower | U.S. government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	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%	0.00%
BB+ and lower | Sovereign debt/sovereign guaranteed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
BB+ and lower | Non-agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	15.00%	14.00%
BB+ and lower | Supranational
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
BB+ and lower | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
A1+/A2/SP-1
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
A1+/A2/SP-1 | Agency residential mortgage-backed securities (“RMBS”)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
A1+/A2/SP-1 | U.S. Treasury
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
A1+/A2/SP-1 | U.S. government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	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%	0.00%
A1+/A2/SP-1 | Sovereign debt/sovereign guaranteed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	2.00%
A1+/A2/SP-1 | Non-agency RMBS
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
A1+/A2/SP-1 | Supranational
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%
A1+/A2/SP-1 | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Ratings (percent)	0.00%	0.00%