Investment Securities - Summary of Unrealized Losses on Available-for-Sale Securities (Details) $ in Millions	Sep. 30, 2021 USD ($) Investment	Dec. 31, 2020 USD ($) Investment
Debt Securities, Available-for-sale [Line Items]
Number of investments in unrealized loss position \| Investment	381	93
Number of investments with unrealized losses greater than 12 months \| Investment	2	0
Fair Value of Investments, Less than 12 months	$ 14,587	$ 4,285
Unrealized Losses, Less than 12 months (less than $1 million at December 31, 2020 for U.S. Treasury AFS)	217	15
Fair Value of Investments, 12 months or longer	81	0
Unrealized Losses, 12 months or longer	4	0
Fair Value of Investments	14,668	4,285
Unrealized Losses	221	15
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair Value of Investments, Less than 12 months	5,172	60
Unrealized Losses, Less than 12 months (less than $1 million at December 31, 2020 for U.S. Treasury AFS)	15	0
Fair Value of Investments, 12 months or longer	0	0
Unrealized Losses, 12 months or longer	0	0
Fair Value of Investments	5,172	60
Unrealized Losses	15	0
U.S. agency debentures
Debt Securities, Available-for-sale [Line Items]
Fair Value of Investments, Less than 12 months	112	133
Unrealized Losses, Less than 12 months (less than $1 million at December 31, 2020 for U.S. Treasury AFS)	4	1
Fair Value of Investments, 12 months or longer	0	0
Unrealized Losses, 12 months or longer	0	0
Fair Value of Investments	112	133
Unrealized Losses	4	1
Residential mortgage-backed securities \| Agency-issued mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value of Investments, Less than 12 months	8,376	904
Unrealized Losses, Less than 12 months (less than $1 million at December 31, 2020 for U.S. Treasury AFS)	187	1
Fair Value of Investments, 12 months or longer	0	0
Unrealized Losses, 12 months or longer	0	0
Fair Value of Investments	8,376	904
Unrealized Losses	187	1
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—fixed rate
Debt Securities, Available-for-sale [Line Items]
Fair Value of Investments, Less than 12 months	232	2,199
Unrealized Losses, Less than 12 months (less than $1 million at December 31, 2020 for U.S. Treasury AFS)	2	10
Fair Value of Investments, 12 months or longer	0	0
Unrealized Losses, 12 months or longer	0	0
Fair Value of Investments	232	2,199
Unrealized Losses	2	10
Agency-issued commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value of Investments, Less than 12 months	695	989
Unrealized Losses, Less than 12 months (less than $1 million at December 31, 2020 for U.S. Treasury AFS)	9	3
Fair Value of Investments, 12 months or longer	81	0
Unrealized Losses, 12 months or longer	4	0
Fair Value of Investments	776	989
Unrealized Losses	$ 13	$ 3

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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