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Amount of cash inflow (outflow) of investing activities of discontinued operations. Investing activity cash flows include making and collecting loans and acquiring and disposing of debt or equity instruments and property, plant, and equipment and other productive assets.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount of equity impact of cash distribution declared to unit-holder of limited partnership (LP).

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Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

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Discontinued Operations - Additional Information (Detail) - USD ($) shares in Millions					1 Months Ended	12 Months Ended
	Nov. 23, 2018	Nov. 23, 2018	Nov. 07, 2018	Sep. 05, 2018	Nov. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 19, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net cash provided by (used in) operating activities from discontinued operations						$ 0	$ (748,231,000)	$ 895,853,000
Net cash provided by (used in) investing activities from discontinued operations						0	18,347,000	(41,027,000)
Revolving Credit Facility [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Repayments of outstanding borrowings			$ 150,000,000.0
Intercompany Credit Agreement [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Line of credit facility, outstanding amount									$ 150,000,000.0
Cantor [Member] \| Revolving Credit Facility [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Line of credit facility, outstanding amount						$ 0	$ 0		$ 150,000,000.0
8.125% Senior Notes Due 2042 [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Stated interest rate						8.125%	8.125%
Debt maturity year						2042
Repayments of outstanding borrowings				$ 112,500,000
Interest expense recorded						$ 0	$ 0	$ 0
5.375% Senior Notes Due 2019 [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Stated interest rate						5.375%
5.375% Senior Notes Due 2019 [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Stated interest rate	5.375%	5.375%
Debt maturity year	2019					2019
Repayments of outstanding borrowings		$ 300,000,000.0
Newmark [Member] \| Intercompany Credit Agreement [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Repayments of outstanding borrowings			$ 150,000,000.0
Newmark [Member] \| 5.375% Senior Notes Due 2019 [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Repayments of outstanding borrowings	$ 300,000,000.0
LPUs [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of limited partnership units granted exchangeability					8.4			6.5
Newmark [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net cash provided by (used in) operating activities from discontinued operations							(748,200,000)	$ 895,900,000
Net cash provided by (used in) investing activities from discontinued operations							18,300,000	(41,000,000.0)
Income allocated to limited partnership units							37,000,000.0
Interest expense recorded							46,100,000	2,700,000
Newmark [Member] \| Intercompany Credit Agreement [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Interest expense recorded							3,500,000	0
Newmark [Member] \| LPUs [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of limited partnership units granted exchangeability					3.9
Equity-based compensation expense							$ 111,100,000	0
BGC Holdings Partnership Unit [Member] \| LPUs [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Equity-based compensation expense								$ 89,400,000