Fair Values of Assets and Liabilities (Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 390,054	$ 337,400
Certificates of deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	999	300
U.S. agency mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	334,355	290,353
Asset-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	17,546	18,898
Corporate debt securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	12,344	12,796
U.S. Treasury securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	21,282	10,656
U.S. state and municipal securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,687	1,697
Non-agency commercial mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,190	1,265
Foreign government agency securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	425	1,413
Commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	200
Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	26	22
Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	11,719	11,159
Investments segregated and on deposit for regulatory purposes:	36,699	31,248
Fair Value	390,054	337,400
Other assets:	1,584	687
Total assets	440,056	380,494
Accrued expenses and other liabilities	1,399
Total liabilities	1,399
Fair Value, Recurring [Member] | Money market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	11,719	11,159
Fair Value, Recurring [Member] | Certificates of deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes:	350	550
Fair Value	999	300
Fair Value, Recurring [Member] | US Government securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes:	36,349	30,698
Other assets:	3	253
Fair Value, Recurring [Member] | U.S. agency mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	334,355	290,353
Fair Value, Recurring [Member] | Asset-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	17,546	18,898
Fair Value, Recurring [Member] | Corporate debt securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	12,344	12,796
Fair Value, Recurring [Member] | U.S. Treasury securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	21,282	10,656
Fair Value, Recurring [Member] | U.S. state and municipal securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,687	1,697
Fair Value, Recurring [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,190	1,265
Fair Value, Recurring [Member] | Foreign government agency securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	425	1,413
Fair Value, Recurring [Member] | Commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	200
Fair Value, Recurring [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	26	22
Fair Value, Recurring [Member] | Equity, corporate debt, and other securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	913	36
Fair Value, Recurring [Member] | Equity and bond mutual funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	636	361
Fair Value, Recurring [Member] | State and municipal debt obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	32	37
Level 1 [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	11,719	11,159
Investments segregated and on deposit for regulatory purposes:	0	0
Fair Value	0	0
Other assets:	1,490	368
Total assets	13,209	11,527
Accrued expenses and other liabilities	1,354
Total liabilities	1,354
Level 1 [Member] | Fair Value, Recurring [Member] | Money market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	11,719	11,159
Level 1 [Member] | Fair Value, Recurring [Member] | Certificates of deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes:	0	0
Fair Value	0	0
Level 1 [Member] | Fair Value, Recurring [Member] | US Government securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes:	0	0
Other assets:	0	0
Level 1 [Member] | Fair Value, Recurring [Member] | U.S. agency mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 1 [Member] | Fair Value, Recurring [Member] | Asset-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 1 [Member] | Fair Value, Recurring [Member] | Corporate debt securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 1 [Member] | Fair Value, Recurring [Member] | U.S. Treasury securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 1 [Member] | Fair Value, Recurring [Member] | U.S. state and municipal securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 1 [Member] | Fair Value, Recurring [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 1 [Member] | Fair Value, Recurring [Member] | Foreign government agency securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 1 [Member] | Fair Value, Recurring [Member] | Commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0
Level 1 [Member] | Fair Value, Recurring [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 1 [Member] | Fair Value, Recurring [Member] | Equity, corporate debt, and other securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	854	7
Level 1 [Member] | Fair Value, Recurring [Member] | Equity and bond mutual funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	636	361
Level 1 [Member] | Fair Value, Recurring [Member] | State and municipal debt obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	0	0
Level 2 [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	0	0
Investments segregated and on deposit for regulatory purposes:	36,699	31,248
Fair Value	390,054	337,400
Other assets:	94	319
Total assets	426,847	368,967
Accrued expenses and other liabilities	45
Total liabilities	45
Level 2 [Member] | Fair Value, Recurring [Member] | Money market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	0	0
Level 2 [Member] | Fair Value, Recurring [Member] | Certificates of deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes:	350	550
Fair Value	999	300
Level 2 [Member] | Fair Value, Recurring [Member] | US Government securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes:	36,349	30,698
Other assets:	3	253
Level 2 [Member] | Fair Value, Recurring [Member] | U.S. agency mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	334,355	290,353
Level 2 [Member] | Fair Value, Recurring [Member] | Asset-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	17,546	18,898
Level 2 [Member] | Fair Value, Recurring [Member] | Corporate debt securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	12,344	12,796
Level 2 [Member] | Fair Value, Recurring [Member] | U.S. Treasury securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	21,282	10,656
Level 2 [Member] | Fair Value, Recurring [Member] | U.S. state and municipal securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,687	1,697
Level 2 [Member] | Fair Value, Recurring [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,190	1,265
Level 2 [Member] | Fair Value, Recurring [Member] | Foreign government agency securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	425	1,413
Level 2 [Member] | Fair Value, Recurring [Member] | Commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	200
Level 2 [Member] | Fair Value, Recurring [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	26	22
Level 2 [Member] | Fair Value, Recurring [Member] | Equity, corporate debt, and other securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	59	29
Level 2 [Member] | Fair Value, Recurring [Member] | Equity and bond mutual funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	0	0
Level 2 [Member] | Fair Value, Recurring [Member] | State and municipal debt obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	32	37
Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities 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
Accrued expenses and other liabilities	0
Total liabilities	0
Level 3 [Member] | Fair Value, Recurring [Member] | Money market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	0	0
Level 3 [Member] | Fair Value, Recurring [Member] | Certificates of deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes:	0	0
Fair Value	0	0
Level 3 [Member] | Fair Value, Recurring [Member] | US Government securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes:	0	0
Other assets:	0	0
Level 3 [Member] | Fair Value, Recurring [Member] | U.S. agency mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 3 [Member] | Fair Value, Recurring [Member] | Asset-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 3 [Member] | Fair Value, Recurring [Member] | Corporate debt securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 3 [Member] | Fair Value, Recurring [Member] | U.S. Treasury securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 3 [Member] | Fair Value, Recurring [Member] | U.S. state and municipal securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 3 [Member] | Fair Value, Recurring [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 3 [Member] | Fair Value, Recurring [Member] | Foreign government agency securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 3 [Member] | Fair Value, Recurring [Member] | Commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0
Level 3 [Member] | Fair Value, Recurring [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Level 3 [Member] | Fair Value, Recurring [Member] | Equity, corporate debt, and other securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	0	0
Level 3 [Member] | Fair Value, Recurring [Member] | Equity and bond mutual funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	0	0
Level 3 [Member] | Fair Value, Recurring [Member] | State and municipal debt obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets:	$ 0	$ 0