Organization and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands								3 Months Ended		9 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 ($)	Sep. 30, 2019 USD ($) shares	Sep. 30, 2018 shares	Sep. 30, 2019 USD ($) Segment shares	Sep. 30, 2018 shares	Nov. 30, 2018 Segment shares	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($) shares	Mar. 19, 2018 USD ($)	Jan. 01, 2018 USD ($)
Description Of Business [Line Items]
Equity method investments								$ 35,600		$ 35,600				$ 35,200
Number of reporting segments | Segment										1		2
Retained deficit								(1,176,741)		$ (1,176,741)				(1,105,019)
Accumulated other comprehensive income (loss)								(33,652)		(33,652)				(24,465)
Noncontrolling interest in subsidiaries								77,364		77,364				$ 94,801
Operating lease, right-of-use asset								168,338		168,338
Operating lease, liability								$ 186,177		$ 186,177
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%				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%				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								53,810	23,767	162,237	25,975,240
Common stock, shares issued | shares								352,840,000		352,840,000				341,745,000
Class A Common Stock [Member] | Newmark [Member]
Description Of Business [Line Items]
Stockholders 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	0	0
Common stock, shares issued | shares								45,884,000		45,884,000				45,884,000
Class B Common Stock [Member] | Newmark [Member]
Description Of Business [Line Items]
Stockholders received on per share	0.463895	0.463895
Common stock, shares issued | shares	21,285,537											21,285,537
Exchange Ratio	0.9362
Real Estate L.P.
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