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	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	$ 281,216	$ 61,422
Commercial paper [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	200	395
Certificates of deposit [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	803	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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	237,087	46,155
Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	22,119	4,987
Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	13,039	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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	4,771	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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	1,620
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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	1,254	13
Foreign government agency securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	301
Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $273,747 at June 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	5,516	7,677
Investments segregated and on deposit for regulatory purposes	21,535	8,627
Available for sale securities (amortized cost of $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	281,216	61,422
Other assets	718	718
Total	308,985	78,444
Fair Value, Recurring [Member] | Money market funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	5,516	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 (amortized cost of $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	200	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	1,378	1,351
Available for sale securities (amortized cost of $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	803	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	20,157	7,276
Other assets	278	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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	237,087	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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	22,119	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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	13,039	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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	4,771	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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	1,620
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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	1,254	13
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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	301
Fair Value, Recurring [Member] | Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $273,747 at June 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	375	442
Fair Value, Recurring [Member] | State and municipal debt obligations [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	30	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	35	27
Fair Value, Recurring [Member] | Level 1 [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	5,516	5,179
Investments segregated and on deposit for regulatory purposes	0	0
Available for sale securities (amortized cost of $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	0	0
Other assets	382	447
Total	5,898	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	5,516	5,179
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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	0	0
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 $273,747 at June 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 $273,747 at June 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 $273,747 at June 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 $273,747 at June 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 $273,747 at June 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 $273,747 at June 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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	0	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 $273,747 at June 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 $273,747 at June 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	375	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	7	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	21,535	8,627
Available for sale securities (amortized cost of $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	281,216	61,422
Other assets	336	271
Total	303,087	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] | 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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	200	395
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	1,378	1,351
Available for sale securities (amortized cost of $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	803	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	20,157	7,276
Other assets	278	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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	237,087	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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	22,119	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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	13,039	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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	4,771	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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	1,620
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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	1,254	13
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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	301
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 $273,747 at June 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	30	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	28	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 $273,747 at June 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] | Commercial paper [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Available for sale securities (amortized cost of $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	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 $273,747 at June 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 $273,747 at June 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 $273,747 at June 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 $273,747 at June 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 $273,747 at June 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 $273,747 at June 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 $273,747 at June 30, 2020 and $61,155 at December 31, 2019)	0	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 $273,747 at June 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 $273,747 at June 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