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	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	$ 303,758	$ 61,422
Certificates of deposit [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	501	1,004
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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	258,266	46,155
Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	19,321	4,987
Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	13,580	5,484
U.S. Treasury securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	7,902	3,384
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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	1,690
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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	1,266	13
Commercial paper [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)		395
Foreign government agency securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	1,210
Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	22
Fair Value, Recurring [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	3,757	7,677
Investments segregated and on deposit for regulatory purposes	14,074	8,627
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	303,758	61,422
Other assets	781	718
Total	322,370	78,444
Fair Value, Recurring [Member] | Money market funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	3,757	5,179
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	901	1,351
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	501	1,004
Fair Value, Recurring [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	13,173	7,276
Other assets	276	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 (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	258,266	46,155
Fair Value, Recurring [Member] | Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	19,321	4,987
Fair Value, Recurring [Member] | Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	13,580	5,484
Fair Value, Recurring [Member] | U.S. Treasury securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	7,902	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 (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	1,690
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 (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	1,266	13
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 (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)		395
Fair Value, Recurring [Member] | Foreign government agency securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	1,210
Fair Value, Recurring [Member] | Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	22
Fair Value, Recurring [Member] | Equity and bond mutual funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	352	442
Fair Value, Recurring [Member] | State and municipal debt obligations [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	22	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	131	27
Fair Value, Recurring [Member] | Level 1 [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	3,757	5,179
Investments segregated and on deposit for regulatory purposes	0	0
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0	0
Other assets	446	447
Total	4,203	5,626
Fair Value, Recurring [Member] | Level 1 [Member] | Money market funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	3,757	5,179
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0	0
Fair Value, Recurring [Member] | 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
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0	0
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0	0
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | U.S. Treasury securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0	0
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Commercial paper [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)		0
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0
Fair Value, Recurring [Member] | Level 1 [Member] | Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0
Fair Value, Recurring [Member] | Level 1 [Member] | Equity and bond mutual funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	352	442
Fair Value, Recurring [Member] | Level 1 [Member] | State and municipal debt obligations [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Equity, corporate debt, and other securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	94	5
Fair Value, Recurring [Member] | Level 2 [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	14,074	8,627
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	303,758	61,422
Other assets	335	271
Total	318,167	72,818
Fair Value, Recurring [Member] | Level 2 [Member] | Money market funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Recurring [Member] | 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	901	1,351
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	501	1,004
Fair Value, Recurring [Member] | 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	13,173	7,276
Other assets	276	202
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	258,266	46,155
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	19,321	4,987
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	13,580	5,484
Fair Value, Recurring [Member] | Level 2 [Member] | U.S. Treasury securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	7,902	3,384
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	1,690
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	1,266	13
Fair Value, Recurring [Member] | Level 2 [Member] | Commercial paper [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		2,498
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)		395
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	1,210
Fair Value, Recurring [Member] | Level 2 [Member] | Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	22
Fair Value, Recurring [Member] | Level 2 [Member] | Equity and bond mutual funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0	0
Fair Value, Recurring [Member] | Level 2 [Member] | State and municipal debt obligations [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	22	47
Fair Value, Recurring [Member] | Level 2 [Member] | Equity, corporate debt, and other securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	37	22
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0	0
Other assets	0	0
Total	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Money market funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0	0
Fair Value, Recurring [Member] | 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
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0	0
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0	0
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | U.S. Treasury securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0	0
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Commercial paper [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)		0
Fair Value, Recurring [Member] | 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 $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0
Fair Value, Recurring [Member] | Level 3 [Member] | Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $296,199 at September 30, 2020 and $61,155 at December 31, 2019)	0
Fair Value, Recurring [Member] | Level 3 [Member] | Equity and bond mutual funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | State and municipal debt obligations [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0	0
Fair Value, Recurring [Member] | 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