Fair Values of Assets and Liabilities - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 101,086	$ 107,646
U.S. agency mortgage-backed securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	60,171	62,795
U.S. Treasury securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	20,193	21,471
Corporate debt securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	11,368	12,484
Asset-Backed Securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,847	9,087
Foreign government agency securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	803	1,002
U.S. state and municipal securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	575	579
Non-agency commercial mortgage-backed securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	110	109
Certificates of deposit
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		100
Other
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	19	19
Fair Value, Recurring
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	12,807	14,573
Investments segregated and on deposit for regulatory purposes:	13,780	20,358
Fair Value	101,086	107,646
Other assets:	2,127	1,913
Total assets	129,800	144,490
Total accrued expenses and other liabilities	1,894	1,733
Total liabilities	1,894	1,733
Fair Value, Recurring | Money market funds
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	12,807	14,573
Fair Value, Recurring | U.S. Government securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes:	13,780	20,358
Other assets:	2	26
Fair Value, Recurring | U.S. agency mortgage-backed securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	60,171	62,795
Fair Value, Recurring | U.S. Treasury securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	20,193	21,471
Fair Value, Recurring | Corporate debt securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	11,368	12,484
Fair Value, Recurring | Asset-Backed Securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,847	9,087
Fair Value, Recurring | Foreign government agency securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	803	1,002
Fair Value, Recurring | U.S. state and municipal securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	575	579
Fair Value, Recurring | Non-agency commercial mortgage-backed securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	110	109
Fair Value, Recurring | Certificates of deposit
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		100
Fair Value, Recurring | Other
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	19	19
Fair Value, Recurring | Total other securities owned
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	2,126	1,913
Fair Value, Recurring | Equity, corporate debt, and other securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	1,228	1,065
Fair Value, Recurring | Mutual funds and ETFs
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	865	795
Fair Value, Recurring | State and municipal debt obligations
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	31	27
Fair Value, Recurring | Interest rate swaps
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	1
Fair Value, Recurring | Other
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Total accrued expenses and other liabilities	1,894	1,733
Fair Value, Recurring | Level 1
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	12,807	14,573
Investments segregated and on deposit for regulatory purposes:	0	0
Fair Value	0	0
Other assets:	2,030	1,787
Total assets	14,837	16,360
Total accrued expenses and other liabilities	1,860	1,644
Total liabilities	1,860	1,644
Fair Value, Recurring | Level 1 | Money market funds
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	12,807	14,573
Fair Value, Recurring | Level 1 | U.S. Government securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes:	0	0
Other assets:	0	0
Fair Value, Recurring | Level 1 | U.S. agency mortgage-backed securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 1 | U.S. Treasury securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 1 | Corporate debt securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 1 | Asset-Backed Securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 1 | Foreign government agency securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 1 | U.S. state and municipal securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 1 | Non-agency commercial mortgage-backed securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 1 | Certificates of deposit
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		0
Fair Value, Recurring | Level 1 | Other
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 1 | Total other securities owned
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	2,030	1,787
Fair Value, Recurring | Level 1 | Equity, corporate debt, and other securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	1,165	992
Fair Value, Recurring | Level 1 | Mutual funds and ETFs
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	865	795
Fair Value, Recurring | Level 1 | State and municipal debt obligations
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	0	0
Fair Value, Recurring | Level 1 | Interest rate swaps
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	0
Fair Value, Recurring | Level 1 | Other
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Total accrued expenses and other liabilities	1,860	1,644
Fair Value, Recurring | Level 2
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	0	0
Investments segregated and on deposit for regulatory purposes:	13,780	20,358
Fair Value	101,086	107,646
Other assets:	97	126
Total assets	114,963	128,130
Total accrued expenses and other liabilities	34	89
Total liabilities	34	89
Fair Value, Recurring | Level 2 | Money market funds
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	0	0
Fair Value, Recurring | Level 2 | U.S. Government securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes:	13,780	20,358
Other assets:	2	26
Fair Value, Recurring | Level 2 | U.S. agency mortgage-backed securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	60,171	62,795
Fair Value, Recurring | Level 2 | U.S. Treasury securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	20,193	21,471
Fair Value, Recurring | Level 2 | Corporate debt securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	11,368	12,484
Fair Value, Recurring | Level 2 | Asset-Backed Securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,847	9,087
Fair Value, Recurring | Level 2 | Foreign government agency securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	803	1,002
Fair Value, Recurring | Level 2 | U.S. state and municipal securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	575	579
Fair Value, Recurring | Level 2 | Non-agency commercial mortgage-backed securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	110	109
Fair Value, Recurring | Level 2 | Certificates of deposit
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		100
Fair Value, Recurring | Level 2 | Other
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	19	19
Fair Value, Recurring | Level 2 | Total other securities owned
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	96	126
Fair Value, Recurring | Level 2 | Equity, corporate debt, and other securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	63	73
Fair Value, Recurring | Level 2 | Mutual funds and ETFs
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	0	0
Fair Value, Recurring | Level 2 | State and municipal debt obligations
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	31	27
Fair Value, Recurring | Level 2 | Interest rate swaps
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	1
Fair Value, Recurring | Level 2 | Other
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Total accrued expenses and other liabilities	34	89
Fair Value, Recurring | Level 3
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	0	0
Investments segregated and on deposit for regulatory purposes:	0	0
Fair Value	0	0
Other assets:	0	0
Total assets	0	0
Total accrued expenses and other liabilities	0	0
Total liabilities	0	0
Fair Value, Recurring | Level 3 | Money market funds
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	0	0
Fair Value, Recurring | Level 3 | U.S. Government securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes:	0	0
Other assets:	0	0
Fair Value, Recurring | Level 3 | U.S. agency mortgage-backed securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 3 | U.S. Treasury securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 3 | Corporate debt securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 3 | Asset-Backed Securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 3 | Foreign government agency securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 3 | U.S. state and municipal securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 3 | Non-agency commercial mortgage-backed securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 3 | Certificates of deposit
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		0
Fair Value, Recurring | Level 3 | Other
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Recurring | Level 3 | Total other securities owned
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	0	0
Fair Value, Recurring | Level 3 | Equity, corporate debt, and other securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	0	0
Fair Value, Recurring | Level 3 | Mutual funds and ETFs
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	0	0
Fair Value, Recurring | Level 3 | State and municipal debt obligations
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	0	0
Fair Value, Recurring | Level 3 | Interest rate swaps
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	0
Fair Value, Recurring | Level 3 | Other
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Total accrued expenses and other liabilities	$ 0	$ 0