Collateralized Transactions (Narrative) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Disclosure Collateralized Transactions [Abstract]
Customers, less allowance for doubtful accounts of $41 and $40 as of June 30, 2018 and December 31, 2017	$ 28,970	$ 29,821
Securities purchased under agreement to resell segregated for regulatory purposes	$ 9,000	$ 9,200
Percentage of securities repledged and deposited for customers	95.00%	82.00%

The carrying value of funds, segregated for regulatory purposes and for the exclusive benefit of customers, outstanding loaned in the form of security resale agreements if the agreement requires the purchaser to resell the identical security purchased or a security that meets the definition of "substantially the same" in the case of a dollar roll. Also includes purchases of participations in pools of securities that are subject to a resale agreement.

No definition available.

Name:	ibkr_SegregatedSecuritiesPurchasedUnderAgreementsToResell
Namespace Prefix:	ibkr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant