Fair Values of Assets and Liabilities (Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - Fair Value, Recurring [Member] - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 11,118	$ 11,159
Investments segregated and on deposit for regulatory purposes	23,117	31,248
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	359,622	337,400
Other assets	173	687
Total	394,030	380,494
Money market funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	11,117	11,159
U.S. Treasury securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	8,829	10,656
Certificates of deposit [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes	350	550
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	1,000	300
U.S. Government securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes	22,767	30,698
Other assets	5	253
U.S. agency mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	314,768	290,353
Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	17,528	18,898
Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	13,093	12,796
U.S. state and municipal securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	1,722	1,697
Foreign government agency securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	1,408	1,413
Non-agency commercial mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	1,248	1,265
Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	26	22
Equity and bond mutual funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	113	361
State and municipal debt obligations [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	21	37
Equity, corporate debt, and other securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	34	36
Level 1 [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	11,117	11,159
Investments segregated and on deposit for regulatory purposes	0	0
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Other assets	128	368
Total	11,245	11,527
Level 1 [Member] | Money market funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	11,117	11,159
Level 1 [Member] | U.S. Treasury securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 1 [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 (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 1 [Member] | U.S. 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] | U.S. agency mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 1 [Member] | Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 1 [Member] | Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 1 [Member] | U.S. state and municipal securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 1 [Member] | Foreign government agency securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 1 [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 1 [Member] | Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 1 [Member] | Equity and bond mutual funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	113	361
Level 1 [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] | Equity, corporate debt, and other securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	15	7
Level 2 [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1	0
Investments segregated and on deposit for regulatory purposes	23,117	31,248
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	359,622	337,400
Other assets	45	319
Total	382,785	368,967
Level 2 [Member] | Money market funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Level 2 [Member] | U.S. Treasury securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	8,829	10,656
Level 2 [Member] | Certificates of deposit [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes	350	550
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	1,000	300
Level 2 [Member] | U.S. Government securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Investments segregated and on deposit for regulatory purposes	22,767	30,698
Other assets	5	253
Level 2 [Member] | U.S. agency mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	314,768	290,353
Level 2 [Member] | Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	17,528	18,898
Level 2 [Member] | Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	13,093	12,796
Level 2 [Member] | U.S. state and municipal securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	1,722	1,697
Level 2 [Member] | Foreign government agency securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	1,408	1,413
Level 2 [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	1,248	1,265
Level 2 [Member] | Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	26	22
Level 2 [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] | State and municipal debt obligations [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	21	37
Level 2 [Member] | Equity, corporate debt, and other securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	19	29
Level 3 [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 (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Other assets	0	0
Total	0	0
Level 3 [Member] | Money market funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Level 3 [Member] | U.S. Treasury securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 3 [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 (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 3 [Member] | U.S. 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] | U.S. agency mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 3 [Member] | Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 3 [Member] | Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 3 [Member] | U.S. state and municipal securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 3 [Member] | Foreign government agency securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 3 [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 3 [Member] | Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $356,439 at June 30, 2021 and $330,248 at December 31, 2020)	0	0
Level 3 [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] | State and municipal debt obligations [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0	0
Level 3 [Member] | Equity, corporate debt, and other securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	$ 0	$ 0