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, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	[1]	$ 337,400	$ 61,422
Commercial paper [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities			395
Certificates of deposit [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		300	1,004
U.S. agency mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		290,353	46,155
Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		12,796	5,484
Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		18,898	4,987
U.S. Treasury securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		10,656	3,384
U.S. state and municipal securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		1,697
Foreign government agency securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		1,413
Non-agency commercial mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		1,265	13
Fair Value, Recurring [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		11,159	7,677
Investments segregated and on deposit for regulatory purposes		31,248	8,627
Available for sale securities		337,400	61,422
Other assets		687	718
Total		380,494	78,444
Fair Value, Recurring [Member] | Money market funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		11,159	5,179
Fair Value, Recurring [Member] | Commercial paper [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents			2,498
Available for sale securities			395
Fair Value, Recurring [Member] | Certificates of deposit [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes		550	1,351
Available for sale securities		300	1,004
Fair Value, Recurring [Member] | US Government securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes		30,698	7,276
Other assets		253	202
Fair Value, Recurring [Member] | U.S. agency mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		290,353	46,155
Fair Value, Recurring [Member] | Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		12,796	5,484
Fair Value, Recurring [Member] | Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		18,898	4,987
Fair Value, Recurring [Member] | U.S. Treasury securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		10,656	3,384
Fair Value, Recurring [Member] | U.S. state and municipal securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		1,697
Fair Value, Recurring [Member] | Foreign government agency securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		1,413
Fair Value, Recurring [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		1,265	13
Fair Value, Recurring [Member] | Equity and bond mutual funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets		361	442
Fair Value, Recurring [Member] | State and municipal debt obligations [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets		37	47
Fair Value, Recurring [Member] | Equity, corporate debt, and other securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets		36	27
Fair Value, Recurring [Member] | Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		22
Level 1 [Member] | Fair Value, Recurring [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		11,159	5,179
Investments segregated and on deposit for regulatory purposes		0	0
Available for sale securities		0	0
Other assets		368	447
Total		11,527	5,626
Level 1 [Member] | Fair Value, Recurring [Member] | Money market funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		11,159	5,179
Level 1 [Member] | Fair Value, Recurring [Member] | Commercial paper [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents			0
Available for sale securities			0
Level 1 [Member] | Fair Value, Recurring [Member] | Certificates of deposit [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes		0	0
Available for sale securities		0	0
Level 1 [Member] | Fair Value, Recurring [Member] | US Government securities [Member]
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
Level 1 [Member] | Fair Value, Recurring [Member] | U.S. agency mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0	0
Level 1 [Member] | Fair Value, Recurring [Member] | Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0	0
Level 1 [Member] | Fair Value, Recurring [Member] | Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0	0
Level 1 [Member] | Fair Value, Recurring [Member] | U.S. Treasury securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0	0
Level 1 [Member] | Fair Value, Recurring [Member] | U.S. state and municipal securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0
Level 1 [Member] | Fair Value, Recurring [Member] | Foreign government agency securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0
Level 1 [Member] | Fair Value, Recurring [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0	0
Level 1 [Member] | Fair Value, Recurring [Member] | Equity and bond mutual funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets		361	442
Level 1 [Member] | Fair Value, Recurring [Member] | State and municipal debt obligations [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets		0	0
Level 1 [Member] | Fair Value, Recurring [Member] | Equity, corporate debt, and other securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets		7	5
Level 1 [Member] | Fair Value, Recurring [Member] | Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0
Level 2 [Member] | Fair Value, Recurring [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0	2,498
Investments segregated and on deposit for regulatory purposes		31,248	8,627
Available for sale securities		337,400	61,422
Other assets		319	271
Total		368,967	72,818
Level 2 [Member] | Fair Value, Recurring [Member] | Money market funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0	0
Level 2 [Member] | Fair Value, Recurring [Member] | Commercial paper [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents			2,498
Available for sale securities			395
Level 2 [Member] | Fair Value, Recurring [Member] | Certificates of deposit [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes		550	1,351
Available for sale securities		300	1,004
Level 2 [Member] | Fair Value, Recurring [Member] | US Government securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes		30,698	7,276
Other assets		253	202
Level 2 [Member] | Fair Value, Recurring [Member] | U.S. agency mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		290,353	46,155
Level 2 [Member] | Fair Value, Recurring [Member] | Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		12,796	5,484
Level 2 [Member] | Fair Value, Recurring [Member] | Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		18,898	4,987
Level 2 [Member] | Fair Value, Recurring [Member] | U.S. Treasury securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		10,656	3,384
Level 2 [Member] | Fair Value, Recurring [Member] | U.S. state and municipal securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		1,697
Level 2 [Member] | Fair Value, Recurring [Member] | Foreign government agency securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		1,413
Level 2 [Member] | Fair Value, Recurring [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		1,265	13
Level 2 [Member] | Fair Value, Recurring [Member] | Equity and bond mutual funds [Member]
Fair Value, Assets 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 Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets		37	47
Level 2 [Member] | Fair Value, Recurring [Member] | Equity, corporate debt, and other securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets		29	22
Level 2 [Member] | Fair Value, Recurring [Member] | Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		22
Level 3 [Member] | Fair Value, Recurring [Member]
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
Available for sale securities		0	0
Other assets		0	0
Total		0	0
Level 3 [Member] | Fair Value, Recurring [Member] | Money market funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0	0
Level 3 [Member] | Fair Value, Recurring [Member] | Commercial paper [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents			0
Available for sale securities			0
Level 3 [Member] | Fair Value, Recurring [Member] | Certificates of deposit [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes		0	0
Available for sale securities		0	0
Level 3 [Member] | Fair Value, Recurring [Member] | US Government securities [Member]
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
Level 3 [Member] | Fair Value, Recurring [Member] | U.S. agency mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0	0
Level 3 [Member] | Fair Value, Recurring [Member] | Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0	0
Level 3 [Member] | Fair Value, Recurring [Member] | Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0	0
Level 3 [Member] | Fair Value, Recurring [Member] | U.S. Treasury securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0	0
Level 3 [Member] | Fair Value, Recurring [Member] | U.S. state and municipal securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0
Level 3 [Member] | Fair Value, Recurring [Member] | Foreign government agency securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0
Level 3 [Member] | Fair Value, Recurring [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		0	0
Level 3 [Member] | Fair Value, Recurring [Member] | Equity and bond mutual funds [Member]
Fair Value, Assets 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 Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets		0	0
Level 3 [Member] | Fair Value, Recurring [Member] | Equity, corporate debt, and other securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets		0	$ 0
Level 3 [Member] | Fair Value, Recurring [Member] | Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities		$ 0

[1] Certain prior year amounts have been reclassified to conform to the current year presentation. See Note 1 for additional information.