Capital Requirements - Net Capital and Net Capital Requirements for Company's Subsidiaries (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019
TD Ameritrade Clearing, Inc. [Member]
Net Capital and Net Capital Requirements For Company's Subsidiaries [Line Items]
Net Capital	$ 3,147,000	$ 3,188,000
Required Net Capital (2% of Aggregate Debit Balances)	493,000	493,000
Net Capital in Excess of Required Net Capital	$ 2,654,000	$ 2,695,000
Ratio of Net Capital to Aggregate Debit Balances	12.77%	12.93%
TD Ameritrade, Inc. [Member]
Net Capital and Net Capital Requirements For Company's Subsidiaries [Line Items]
Net Capital	$ 346,000	$ 289,000
Required Net Capital (2% of Aggregate Debit Balances)	250	250
Net Capital in Excess of Required Net Capital	345,000	289,000
TD Ameritrade Futures & Forex LLC [Member]
Net Capital and Net Capital Requirements For Company's Subsidiaries [Line Items]
Adjusted Net Capital	148,000	140,000
Required Adjusted Net Capital ($20 Million Plus 5% of All Foreign Exchange Liabilities Owed to Forex Clients in Excess of $10 Million)	23,000	23,000
Adjusted Net Capital in Excess of Required Adjusted Net Capital	$ 125,000	$ 117,000