Capital Requirements - Additional Information (Details) - USD ($)	Dec. 31, 2019	Sep. 30, 2019
Net Capital and Net Capital Requirements For Company's Subsidiaries [Line Items]
Alternative net capital requirement, percentage of net capital to aggregate debit balances required for a broker-dealer to repay subordinated borrowings, pay cash dividends, or make any unsecured advances or loans to its parent company or employees	5.00%
Alternative net capital requirement, percentage of net capital to the minimum dollar requirement required for a broker-dealer to repay subordinated borrowings, pay cash dividends, or make any unsecured advances or loans to its parent company or employees	120.00%
TD Ameritrade Clearing, Inc. [Member]
Net Capital and Net Capital Requirements For Company's Subsidiaries [Line Items]
Alternative net capital requirement, dollar amount threshold	$ 1,500,000
Alternative net capital requirement, percentage of aggregate debit balances	2.00%
TD Ameritrade, Inc. [Member]
Net Capital and Net Capital Requirements For Company's Subsidiaries [Line Items]
Alternative net capital requirement, dollar amount threshold	$ 250,000
Alternative net capital requirement, percentage of aggregate debit balances	2.00%
TD Ameritrade Futures & Forex LLC [Member]
Net Capital and Net Capital Requirements For Company's Subsidiaries [Line Items]
FCM required net capital under the Commodity Exchange Act, dollar amount threshold	$ 1,000,000.0
FCM required net capital under the Commodity Exchange Act, percentage of total risk margin requirements for all positions carried in client accounts	8.00%
FCM required net capital under the Commodity Exchange Act, percentage of total risk margin requirements for all positions carried in non-client accounts	8.00%
FDM required net capital under the Commodity Exchange Act, dollar amount threshold	$ 20,000,000.0
FDM required net capital under the Commodity Exchange Act, percentage of forex liabilities in excess of $10 million added to the dollar amount threshold	5.00%
FDM required net capital under the Commodity Exchange Act, minimum forex client liability dollar amount threshold	$ 10,000,000.0
FCM and FDM required net capital under the Commodity Exchange Act, required notice to CFTC, minimum percentage of adjusted net capital to the risk-based capital requirement	110.00%
FCM and FDM required net capital under the Commodity Exchange Act, required notice to CFTC, minimum percentage of adjusted net capital to the FCM's $1 million threshold	150.00%
FCM and FDM required net capital under the Commodity Exchange Act, required notice to CFTC, minimum percentage of adjusted net capital to the FDM's $20 million plus 5% of foreign exchange liabilities owed to forex clients in excess of $10 million threshold	110.00%
TD Ameritrade Trust Company [Member]
Net Capital and Net Capital Requirements For Company's Subsidiaries [Line Items]
Tier 1 capital, balance at period end	$ 43,000,000	$ 43,000,000
Amount in excess of required Tier 1 capital	$ 26,000,000	$ 22,000,000