Organization and Basis of Presentation - Additional Information (Detail)							3 Months Ended		12 Months Ended
	Mar. 19, 2018 USD ($)	Mar. 07, 2018 USD ($) shares	Dec. 26, 2017 USD ($) shares	Dec. 15, 2017 $ / shares shares	Nov. 22, 2017 USD ($)	Sep. 08, 2017 USD ($) shares	Mar. 31, 2018 USD ($) Segment shares	Mar. 31, 2017 USD ($) shares	Dec. 31, 2017 USD ($) shares	Mar. 18, 2018 USD ($)	Jan. 01, 2018 USD ($)	Apr. 21, 2017 USD ($)	Dec. 09, 2014 USD ($)	Jun. 26, 2012 USD ($)
Description Of Business [Line Items]
Number of servicing segments | Segment							2
Total consideration transferred									$ 1,000,000,000
Equity method investments							$ 139,700	$ 135,600
Fully diluted weighted-average shares of common stock outstanding | shares							478,935,000	444,826,000
Expected repayment of debt owed							$ 946,258,000		946,930,000
Assets							6,777,496,000		5,429,712,000
Liabilities							5,290,789,000		4,243,556,000
Noncontrolling interest in subsidiaries							561,353,000		505,855,000
Retained deficit							(837,753,000)		(859,009,000)
Accumulated other comprehensive income (loss)							$ (8,754,000)		$ (10,486,000)
ASU No. 2014-09 [Member]
Description Of Business [Line Items]
Assets											$ 64,400,000
Liabilities											41,900,000
Noncontrolling interest in subsidiaries											2,300,000
Retained deficit											20,200,000
ASU No. 2014-09 [Member] | Retained Deficit [Member]
Description Of Business [Line Items]
Tax effect impact for increase or decrease to the liability											3,900,000
ASU No. 2014-09 [Member] | Deferred Tax Liability [Member]
Description Of Business [Line Items]
Tax effect impact for increase or decrease to the liability											3,900,000
ASU No. 2018-03 [Member]
Description Of Business [Line Items]
Noncontrolling interest in subsidiaries											800,000
Retained deficit											2,100,000
Accumulated other comprehensive income (loss)											2,900,000
ASU No. 2018-03 [Member] | Retained Deficit [Member]
Description Of Business [Line Items]
Tax effect impact for increase or decrease to the liability											400,000
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											600,000
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											$ 200,000
Cantor [Member]
Description Of Business [Line Items]
Noncontrolling interest, ownership percentage							48.00%
Line of credit facility, outstanding amount						$ 150,000,000	$ 0
Maximum revolving credit												$ 150,000,000
Revolving Credit Facility [Member] | Cantor [Member]
Description Of Business [Line Items]
Principal amount of notes	$ 250,000,000									$ 150,000
Stated interest rate							5.13%
Line of credit facility, outstanding amount							$ 180,000,000
Credit agreement, description							The BGC Credit Agreement will mature on the earlier to occur of (a) March 19, 2019, after which the maturity date of the BGC Credit Agreement will continue to be extended for successive one-year periods unless prior written notice of non-extension is given by a lending party to a borrowing party at least six months in advance of such renewal date and (b) the termination of the BGC Credit Agreement by either party pursuant to its terms.
Credit agreement, interest rate description							The outstanding amounts under the BGC Credit Agreement will bear interest for any rate period at a per annum rate equal to the higher of BGC’s or Cantor’s short-term borrowing rate in effect at such time plus 1.00%
Interest rate	1.00%
Line of credit facility, borrowing amount							$ 180,000,000
8.125% Senior Notes [Member]
Description Of Business [Line Items]
Principal amount of notes														$ 112,500,000
Stated interest rate							8.125%		8.125%
Debt maturity year							2042
Expected repayment of debt owed							$ 109,427,000		$ 109,396,000
5.375% Senior Notes [Member]
Description Of Business [Line Items]
Principal amount of notes													$ 300,000,000
Stated interest rate							5.375%		5.375%
Debt maturity year							2019
Expected repayment of debt owed							$ 298,310,000		$ 298,064,000
Term Loan Credit Agreement [Member]
Description Of Business [Line Items]
Principal amount of notes					$ 575,000,000
Net proceeds from the IPO to partially repay the term loan					304,300,000
Line of credit facility, outstanding amount									$ 270,700,000
Revolver Amendment [Member]
Description Of Business [Line Items]
Line of credit facility, outstanding amount					400,000,000
Maximum revolving credit					$ 400,000,000
Revolving Credit Agreement [Member]
Description Of Business [Line Items]
Stated interest rate	4.99%
Line of credit facility, outstanding amount							$ 202,000,000
Interest rate	1.00%
Line of credit facility, borrowing amount	$ 150,000,000
Class A Common Stock [Member]
Description Of Business [Line Items]
Issuance of Class A common stock (net of costs), shares | shares							18,925,702		1,994,670
Common stock, shares outstanding | shares							279,279,000		256,968,000
IPO [Member]
Description Of Business [Line Items]
Net proceeds from the IPO to partially repay the term loan							$ 304,300,000
CF Real Estate Finance Holdings, L.P. [Member]
Description Of Business [Line Items]
Equity method investments						$ 100,000,000
BPF [Member]
Description Of Business [Line Items]
Business acquisition date						Jul. 17, 2017
Total consideration transferred						$ 875,000,000
Business acquisition price paid in units						3,200,000
Business acquisition post closing adjustments						$ 508,600,000
Business acquisition date						Sep. 08, 2017
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) | shares						247,099
Newmark [Member]
Description Of Business [Line Items]
Expected repayment of debt owed							$ 812,500
Newmark [Member] | IPO [Member]
Description Of Business [Line Items]
Noncontrolling interest, ownership percentage			9.80%
Fully diluted weighted-average shares of common stock outstanding | shares			234,200,000
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
Option to purchase additional shares | shares			3,000,000
Proceeds from Newmark initial public offering			$ 295,400,000
Noncontrolling interest, ownership percentage			16.60%
Common stock, shares outstanding | shares			138,600,000
Options to purchase additional shares of common stock period				30 days
Newmark [Member] | Limited Partnership Units [Member]
Description Of Business [Line Items]
Aggregate exchangeable limited partnership units purchased | shares		16,600,000
Aggregate consideration amount		$ 242,000,000