Fair Value Disclosures - Fair Value Hierarchy for Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019
Assets:
Securities owned	$ 520	$ 532
Investments available-for-sale, at fair value	1,636	1,668
Recurring [Member]
Assets:
Investments segregated for regulatory purposes	5,543	5,712
Securities owned	520	532
Other assets	48	72
Total assets at fair value	9,686	10,470
Liabilities:
Total liabilities at fair value	18
Recurring [Member] | U.S. Government Debt Securities [Member]
Assets:
Investments segregated for regulatory purposes	3,772	4,394
Securities owned	513	524
Investments available-for-sale, at fair value	1,636	1,668
Other assets		1
Recurring [Member] | U.S. Government Agency Mortgage-Backed Securities [Member]
Assets:
Investments segregated for regulatory purposes	1,771	1,318
Recurring [Member] | Other Securities [Member]
Assets:
Securities owned	7	8
Other assets	3
Recurring [Member] | Pay Variable Interest Rate Swap [Member]
Assets:
Other assets	45	71
Liabilities:
Accounts payable and other liabilities	17	3
Recurring [Member] | Equity Securities [Member]
Liabilities:
Accounts payable and other liabilities	1
Recurring [Member] | Money Market Mutual Funds [Member]
Assets:
Cash equivalents	1,939	2,486
Recurring [Member] | Level 1 [Member]
Assets:
Securities owned	1	2
Other assets	2
Total assets at fair value	1,942	2,488
Liabilities:
Total liabilities at fair value	1
Recurring [Member] | Level 1 [Member] | Other Securities [Member]
Assets:
Securities owned	1	2
Other assets	2
Recurring [Member] | Level 1 [Member] | Equity Securities [Member]
Liabilities:
Accounts payable and other liabilities	1
Recurring [Member] | Level 1 [Member] | Money Market Mutual Funds [Member]
Assets:
Cash equivalents	1,939	2,486
Recurring [Member] | Level 2 [Member]
Assets:
Investments segregated for regulatory purposes	5,543	5,712
Securities owned	519	530
Other assets	46	72
Total assets at fair value	7,744	7,982
Liabilities:
Total liabilities at fair value	17
Recurring [Member] | Level 2 [Member] | U.S. Government Debt Securities [Member]
Assets:
Investments segregated for regulatory purposes	3,772	4,394
Securities owned	513	524
Investments available-for-sale, at fair value	1,636	1,668
Other assets		1
Recurring [Member] | Level 2 [Member] | U.S. Government Agency Mortgage-Backed Securities [Member]
Assets:
Investments segregated for regulatory purposes	1,771	1,318
Recurring [Member] | Level 2 [Member] | Other Securities [Member]
Assets:
Securities owned	6	6
Other assets	1
Recurring [Member] | Level 2 [Member] | Pay Variable Interest Rate Swap [Member]
Assets:
Other assets	45	71
Liabilities:
Accounts payable and other liabilities	$ 17	$ 3