Long-term Debt and Other Borrowings - Schedule of Long-Term Debt and Other Borrowings (Details) - USD ($)	Jun. 30, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Face Value	$ 3,550,000,000	$ 3,550,000,000
Unamortized Discounts and Debt Issuance Costs	(21,000,000)	(24,000,000)
Fair Value Adjustment	207,000,000	68,000,000
Net Carrying Value	3,736,000,000	3,594,000,000
Senior Notes [Member]
Debt Instrument [Line Items]
Face Value	3,550,000,000	3,550,000,000
Senior Notes [Member] \| Variable-rate Notes due 2021 [Member]
Debt Instrument [Line Items]
Face Value	600,000,000	600,000,000
Unamortized Discounts and Debt Issuance Costs	(2,000,000)	(2,000,000)
Fair Value Adjustment	0	0
Net Carrying Value	598,000,000	598,000,000
Senior Notes [Member] \| 2.950% Notes due 2022 [Member]
Debt Instrument [Line Items]
Face Value	750,000,000	750,000,000
Unamortized Discounts and Debt Issuance Costs	(2,000,000)	(3,000,000)
Fair Value Adjustment	22,000,000	6,000,000
Net Carrying Value	$ 770,000,000	$ 753,000,000
Interest rate, stated percentage	2.95%	2.95%
Senior Notes [Member] \| 3.75% Notes due 2024 [Member]
Debt Instrument [Line Items]
Face Value	$ 400,000,000	$ 400,000,000
Unamortized Discounts and Debt Issuance Costs	(3,000,000)	(3,000,000)
Fair Value Adjustment	0	0
Net Carrying Value	$ 397,000,000	$ 397,000,000
Interest rate, stated percentage	3.75%	3.75%
Senior Notes [Member] \| 3.625% Notes due 2025 [Member]
Debt Instrument [Line Items]
Face Value	$ 500,000,000	$ 500,000,000
Unamortized Discounts and Debt Issuance Costs	(2,000,000)	(3,000,000)
Fair Value Adjustment	50,000,000	25,000,000
Net Carrying Value	$ 548,000,000	$ 522,000,000
Interest rate, stated percentage	3.625%	3.625%
Senior Notes [Member] \| 3.300% Notes due 2027 [Member]
Debt Instrument [Line Items]
Face Value	$ 800,000,000	$ 800,000,000
Unamortized Discounts and Debt Issuance Costs	(7,000,000)	(8,000,000)
Fair Value Adjustment	94,000,000	40,000,000
Net Carrying Value	$ 887,000,000	$ 832,000,000
Interest rate, stated percentage	3.30%	3.30%
Senior Notes [Member] \| 2.750% Notes due 2029 [Member]
Debt Instrument [Line Items]
Face Value	$ 500,000,000	$ 500,000,000
Unamortized Discounts and Debt Issuance Costs	(5,000,000)	(5,000,000)
Fair Value Adjustment	41,000,000	(3,000,000)
Net Carrying Value	$ 536,000,000	$ 492,000,000
Interest rate, stated percentage	2.75%	2.75%

TD AMERITRADE HOLDING CORP 10-Q