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Organization and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands								3 Months Ended		11 Months Ended
	Nov. 30, 2018 shares	Nov. 23, 2018 USD ($)	Nov. 07, 2018 USD ($)	Sep. 05, 2018 USD ($)	Dec. 26, 2017 shares	Dec. 15, 2017 $ / shares shares	Sep. 08, 2017 USD ($)	Mar. 31, 2019 USD ($) Segment shares	Mar. 31, 2018 shares	Nov. 30, 2018 Segment shares	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($) shares	Mar. 19, 2018 USD ($)	Jan. 01, 2018 USD ($)
Description Of Business [Line Items]
Equity method investments								$ 36,400				$ 35,200
Number of reporting segments \| Segment								1		2
Retained deficit								$ (1,090,585)				(1,105,019)
Accumulated other comprehensive income (loss)								(23,553)				(24,465)
Noncontrolling interest in subsidiaries								94,063				$ 94,801
Operating lease, right-of-use asset								190,219
Operating lease, liability								$ 204,889
ASU No. 2018-03 [Member]
Description Of Business [Line Items]
Retained deficit														$ 2,100
Accumulated other comprehensive income (loss)														2,900
Noncontrolling interest in subsidiaries														800
ASU No. 2018-03 [Member] \| Retained Deficit [Member]
Description Of Business [Line Items]
Tax effect impact for increase or decrease to the liability														400
ASU No. 2018-03 [Member] \| Accumulated Other Comprehensive Income (Loss) [Member]
Description Of Business [Line Items]
Tax effect impact for increase or decrease to the liability														600
ASU No. 2018-03 [Member] \| Noncontrolling Interest in Subsidiaries [Member]
Description Of Business [Line Items]
Tax effect impact for increase or decrease to the liability														$ 200
ASU No. 2016-02 [Member]
Description Of Business [Line Items]
Operating lease, right-of-use asset											$ 192,400
Operating lease, liability											$ 206,000
BGC Holdings Partnership Unit [Member]
Description Of Business [Line Items]
Exchangeable limited partnership units \| shares	1,458,931
Newmark [Member]
Description Of Business [Line Items]
Percentage of voting power	94.00%									94.00%
Percentage of economics of outstanding common stock	87.00%									87.00%
Revolving Credit Agreement [Member]
Description Of Business [Line Items]
Line of credit facility, borrowing amount													$ 150,000
Intercompany Credit Agreement [Member]
Description Of Business [Line Items]
Line of credit facility, borrowing amount													$ 150,000
8.125% Senior Notes Due 2042 [Member]
Description Of Business [Line Items]
Stated interest rate				8.125%				8.125%				8.125%
Debt maturity year				2042				2042
Repayments of outstanding borrowings				$ 112,500
5.375% Senior Notes Due 2019 [Member]
Description Of Business [Line Items]
Stated interest rate		5.375%						5.375%				5.375%
Debt maturity year		2019						2019
Repayments of outstanding borrowings		$ 300,000
Class A Common Stock [Member]
Description Of Business [Line Items]
Issuance of Class A common stock (net of costs), shares \| shares								61,642	18,925,702
Common stock, shares issued \| shares								343,885,000				341,745,000
Class A Common Stock [Member] \| Newmark [Member]
Description Of Business [Line Items]
Spin-off received on per share	0.463895	0.463895
Common stock, shares issued \| shares	131,886,409									131,886,409
Class A Common Stock [Member] \| Newmark [Member] \| Cantor [Member]
Description Of Business [Line Items]
Common stock, shares issued \| shares	449,917									449,917
Class B Common Stock [Member]
Description Of Business [Line Items]
Issuance of Class A common stock (net of costs), shares \| shares								0	0
Common stock, shares issued \| shares								45,884,000				45,884,000
Class B Common Stock [Member] \| Newmark [Member]
Description Of Business [Line Items]
Spin-off received on per share	0.463895	0.463895
Common stock, shares issued \| shares	21,285,537									21,285,537
Exchange ratio	0.9558
CF Real Estate Finance Holdings, L.P. [Member]
Description Of Business [Line Items]
Equity method investments							$ 100,000
BPF [Member]
Description Of Business [Line Items]
Business acquisition date							Jul. 17, 2017
Business acquisition effective date							Sep. 08, 2017
Newmark [Member] \| Intercompany Credit Agreement [Member]
Description Of Business [Line Items]
Repayments of outstanding borrowings			$ 150,000
Newmark [Member] \| 8.125% Senior Notes Due 2042 [Member]
Description Of Business [Line Items]
Repayments of outstanding borrowings				$ 112,500
Newmark [Member] \| 5.375% Senior Notes Due 2019 [Member]
Description Of Business [Line Items]
Repayments of outstanding borrowings		$ 300,000
Newmark [Member] \| IPO [Member] \| Class A Common Stock [Member]
Description Of Business [Line Items]
Issuance of Class A common stock (net of costs), shares \| shares						20,000,000
Closing price per share \| $ / shares						$ 14.00
Options to purchase additional shares of common stock period						30 days
Option to purchase additional shares \| shares					3,000,000