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Derivatives - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 13, 2017
Option Indexed to Issuer's Equity [Line Items]
Replacement cost of contracts in a gain position	$ 24.7		$ 12.7
Change in fair value of rate lock commitments, net	15.1	$ 2.1
Rate Lock Commitments [Member]
Option Indexed to Issuer's Equity [Line Items]
Change in fair value of rate lock commitments, net	$ 2.5	$ 0.8

X

- Definition

The replacement cost of derivative contracts in a gain or asset position.

+ References

No definition available.

+ Details

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Namespace Prefix:	bgcp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

For each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14172-108612

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: