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Equity And Earnings Per Share (IBG LLC Ownership of Member Interests) (Details)	Sep. 30, 2018 shares
Ownership Percentage	100.00%
Membership Interests	413,789,160
IBG Inc [Member]
Ownership Percentage	18.10%
Membership Interests	75,097,443
Holdings [Member]
Ownership Percentage	81.90%
Membership Interests	338,691,717

X

- Definition

The amount of membership interests in IBG LLC

+ References

No definition available.

+ Details

Name:	ibkr_MembershipInterests
Namespace Prefix:	ibkr_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

X

- Definition

The percent of ownership in IBG LLC membership interests

+ References

No definition available.

+ Details

Name:	ibkr_OwnershipPercentage
Namespace Prefix:	ibkr_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	dei_LegalEntityAxis=ibkr_IbgIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=ibkr_HoldingsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: