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Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Parenthetical) (Detail) - shares shares in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued	1.6	1.4
Limited Partnership [Member]
Class of Stock [Line Items]
Number of units redeemed and cancelled for exchange	1.7	1.4

X

- Definition

Number of units redeemed and cancelled for exchange.

+ References

No definition available.

+ Details

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Namespace Prefix:	bgcp_
Data Type:	xbrli:sharesItemType
Balance Type:	na
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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Period Type:	duration

X

- Definition

The number of shares issued during the period upon the conversion of units. An example of a convertible unit is an umbrella partnership real estate investment trust unit (UPREIT unit).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

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-Name Accounting Standards Codification
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-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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- Details

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