Commitments and Contingencies - Additional Information (Details)		3 Months Ended	12 Months Ended
	Jul. 09, 2019 USD ($)	Dec. 31, 2019 USD ($) investor	Sep. 30, 2014 litigation_case	Sep. 30, 2019 USD ($)	May 31, 2019 USD ($)
Contingencies and Commitments [Line Items]
Contingent liability for guarantees to securities clearinghouses and exchanges		$ 0		$ 0
Order Routing Matters [Member]
Contingencies and Commitments [Line Items]
Number of claims previously filed | litigation_case			6
Number of claims dismissed | litigation_case			5
Number of pending claims | litigation_case			1
Aequitas Securities Litigation [Member]
Contingencies and Commitments [Line Items]
Alleged amount of securities purchased by Company clients		140,000,000
Minimum [Member]
Contingencies and Commitments [Line Items]
Aggregate range of reasonably possible losses in excess of amounts accrued		0
Maximum [Member]
Contingencies and Commitments [Line Items]
Aggregate range of reasonably possible losses in excess of amounts accrued		$ 70,000,000
TD Ameritrade, Inc. [Member] | Order Routing Matters [Member]
Contingencies and Commitments [Line Items]
Number of claims previously filed | litigation_case			5
Scottrade, Inc. [Member] | Order Routing Matters [Member]
Contingencies and Commitments [Line Items]
Number of claims previously filed | litigation_case			1
Ciuffitelli Class Action [Member] | Aequitas Securities Litigation [Member]
Contingencies and Commitments [Line Items]
Alleged number of investors involved in the claim (more than) | investor		1,500
Alleged amount owed to investors		$ 600,000,000
Settlement between plaintiffs and Tonkon Torp Defendant	$ 220,000,000				$ 14,600,000
Amount of settlement to be contributed by the Company	20,000,000
Amount of settlement to be contributed by insurers	$ 12,000,000