Organization and Basis of Presentation - Additional Information (Detail) - USD ($)							3 Months Ended	12 Months Ended
	Mar. 19, 2018	Mar. 07, 2018	Dec. 26, 2017	Dec. 15, 2017	Nov. 22, 2017	Sep. 08, 2017	Mar. 31, 2018	Dec. 31, 2017	Jan. 01, 2018	Dec. 09, 2014	Jun. 26, 2012
Description Of Business [Line Items]
Total consideration transferred								$ 55,600,000
Equity method investments							$ 104,718,000	101,562,000
Fully diluted weighted-average shares of common stock outstanding							246,834,000
Assets							$ 3,057,150,000	2,273,007,000
Liabilities							2,503,468,000	2,029,593,000
Retained earnings							238,096,000	199,492,000
Noncontrolling interests							237,460,000	$ (38,092,000)
ASU No. 2014-09 [Member] | Impact of ASC 606 [Member]
Description Of Business [Line Items]
Assets									$ 64,400,000
Liabilities									45,600,000
Retained earnings									16,500,000
Noncontrolling interests									$ 2,300,000
Intercompany Credit Agreement [Member]
Description Of Business [Line Items]
Line of credit facility, outstanding amount							$ 202,000,000
Stated interest rate							4.99%
Interest rate	1.00%
Intercompany Credit Agreement [Member] | Interest Income, Net [Member]
Description Of Business [Line Items]
Interest expense							$ 1,000,000
Term Loan Credit Agreement [Member]
Description Of Business [Line Items]
Expected repayment of long-term debt owed							0
Principal amount of notes					$ 575,000,000
Net proceeds from the IPO to partially repay the term loan					304,300,000
Repayments of debt							$ 270,700,000
8.125% Senior Notes [Member]
Description Of Business [Line Items]
Principal amount of notes											$ 112,500,000
Stated interest rate							8.125%
Debt maturity year							2042
5.375% Senior Notes [Member]
Description Of Business [Line Items]
Principal amount of notes										$ 300,000,000
Stated interest rate							5.375%
Debt maturity year							2019
Class A Common Stock [Member]
Description Of Business [Line Items]
Stock issued during period, shares, new issues								23,000,000
Common stock, shares outstanding							138,922,000	138,594,000
IPO [Member]
Description Of Business [Line Items]
Noncontrolling interest, ownership percentage			9.80%
Fully diluted weighted-average shares of common stock outstanding			234,200,000
IPO [Member] | Class A Common Stock [Member]
Description Of Business [Line Items]
Stock issued during period, shares, new issues				20,000,000
Closing price per share				$ 14.00
Options to purchase additional shares of common stock period				30 days
Option to purchase additional shares			3,000,000
Proceeds from Newmark initial public offering, net of underwriting discounts and commissions			$ 295,400,000
Noncontrolling interest, ownership percentage			16.60%
Common stock, shares outstanding			138,600,000
Newmark Units [Member]
Description Of Business [Line Items]
Expected repayment of long-term debt owed							$ 812,500,000
Newmark Units [Member] | BGC Partners LP and its Operating Subsidiaries [Member]
Description Of Business [Line Items]
Closing price per share		$ 14.57
Purchase of units		16,600,000
Purchase value of units		$ 242,000,000
BPF [Member]
Description Of Business [Line Items]
Business acquisition date						Sep. 08, 2017
Ownership percentage acquired						100.00%
Total consideration transferred						$ 875,000,000
Business acquisition price paid in units						3,200,000
Business acquisition post closing adjustments						$ 508,600,000
Number of trading days prior to closing						3 days
BPF [Member] | Partnership Units [Member]
Description Of Business [Line Items]
Business acquisition price paid (Partnership Units)						247,099
CF Real Estate Finance Holdings, L.P. [Member]
Description Of Business [Line Items]
Equity method investments						$ 100,000,000
Revolver Amendment [Member]
Description Of Business [Line Items]
Maximum revolving credit					400,000,000
Line of credit facility, outstanding amount					$ 400,000,000