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	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	$ 221,232	$ 61,422
Commercial paper [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	396	395
Certificates of deposit [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	998	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	179,595	46,155
Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	22,481	4,987
Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	11,222	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	3,664	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	1,578
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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	1,226	13
Foreign government agency securities [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	51
Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	21
Fair Value, Recurring [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	9,209	7,677
Investments segregated and on deposit for regulatory purposes	14,598	8,627
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	221,232	61,422
Other assets	695	718
Total	245,734	78,444
Fair Value, Recurring [Member] | Money market funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	5,152	5,179
Fair Value, Recurring [Member] | Commercial paper [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	4,057	2,498
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	396	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	1,949	1,351
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	998	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	12,649	7,276
Other assets	251	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	179,595	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	22,481	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	11,222	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	3,664	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	1,578
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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	1,226	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	51
Fair Value, Recurring [Member] | Other [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	21
Fair Value, Recurring [Member] | Equity and bond mutual funds [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	379	442
Fair Value, Recurring [Member] | State and municipal debt obligations [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	41	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	24	27
Fair Value, Recurring [Member] | Level 1 [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	5,152	5,179
Investments segregated and on deposit for regulatory purposes	0	0
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	0	0
Other assets	382	447
Total	5,534	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	5,152	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	0
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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	379	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	3	5
Fair Value, Recurring [Member] | Level 2 [Member]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	4,057	2,498
Investments segregated and on deposit for regulatory purposes	14,598	8,627
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	221,232	61,422
Other assets	313	271
Total	240,200	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	4,057	2,498
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	396	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	1,949	1,351
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	998	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	12,649	7,276
Other assets	251	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	179,595	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	22,481	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	11,222	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	3,664	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	1,578
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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	1,226	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	51
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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	21
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	41	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	21	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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	0
Available for sale securities (amortized cost of $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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 $215,873 and $61,155 at March 31, 2020 and December 31, 2019, respectively)	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