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Organization and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands |  |  |  |  |  |  | 11 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 30, 2018 shares | Nov. 23, 2018 USD ($) | Nov. 23, 2018 USD ($) | Nov. 07, 2018 USD ($) | Sep. 05, 2018 USD ($) | Dec. 26, 2017 shares | Dec. 15, 2017 $ / shares shares | Nov. 30, 2018 Segment shares | Dec. 31, 2020 USD ($) Individual Segment shares | Dec. 31, 2019 USD ($) shares | Dec. 31, 2018 shares | Jan. 01, 2020 USD ($) | Jan. 01, 2019 USD ($) | Mar. 19, 2018 USD ($) | Jan. 01, 2018 USD ($)
Description Of Business [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of individuals included in perpetrated | Individual |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |
Cumulative impact on net income (loss) as a result of theft |  |  |  |  |  |  |  |  |  | $ 35,200 |  |  |  |  |  |
Number of reporting segments | Segment |  |  |  |  |  |  |  |  | 2 | 1 |  |  |  |  |  |
Retained deficit |  |  |  |  |  |  |  |  |  | $ (1,265,504) | $ (1,253,089) |  |  |  |  | $ (866,875)
Accumulated other comprehensive income (loss) |  |  |  |  |  |  |  |  |  | (28,930) | (33,102) |  |  |  |  |
Noncontrolling interest in subsidiaries |  |  |  |  |  |  |  |  |  | 59,290 | 50,321 |  |  |  |  | 501,208
Operating lease, right-of-use asset | [1] |  |  |  |  |  |  |  |  | 165,969 | 169,065 |  |  |  |  |
Operating lease, liability | [1] |  |  |  |  |  |  |  |  | 190,207 | $ 187,398 |  |  |  |  |
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] | Revision of Prior Period, Change in Accounting Principle, Adjustment [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] | Revision of Prior Period, Change in Accounting Principle, Adjustment [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] | Revision of Prior Period, Change in Accounting Principle, Adjustment [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 |  |
ASU No. 2016-13 [Member] | Revision of Prior Period, Change in Accounting Principle, Adjustment [Member] | Assets [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tax effect impact for increase or decrease to the liability |  |  |  |  |  |  |  |  |  | 600 |  |  | $ 1,900 |  |  |
ASU No. 2016-13 [Member] | Retained Deficit [Member] | Revision of Prior Period, Change in Accounting Principle, Adjustment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tax effect impact for increase or decrease to the liability |  |  |  |  |  |  |  |  |  | 400 |  |  | 1,300 |  |  |
ASU No. 2016-13 [Member] | Noncontrolling Interest in Subsidiaries [Member] | Revision of Prior Period, Change in Accounting Principle, Adjustment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tax effect impact for increase or decrease to the liability |  |  |  |  |  |  |  |  |  | $ 200 |  |  | $ 600 |  |  |
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% |  |  |  |  |  |  |  |  |  |
Debt maturity year |  |  |  |  |  | 2042 |  |  |  |  |  |  |  |  |  |
5.375% Senior Notes Due 2019 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  | 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 |  |  |  |  |  |  |  |  |  | 390,570 | 212,711 | 26,003,424 |  |  |  |
Common stock, shares issued | shares |  |  |  |  |  |  |  |  |  | 373,545,000 | 358,440,000 |  |  |  |  |
Class A Common Stock [Member] | Newmark [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders received on per share |  | 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] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders received on per share |  | 0.463895 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued | shares |  | 21,285,537 |  |  |  |  |  |  | 21,285,537 |  |  |  |  |  |  |
Exchange ratio |  |  |  |  |  |  |  |  |  | 0.9379 |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |
[1] | As of December 31, 2020, $166.0 million and $190.2 million, relating to lease ROU assets and lease liabilities, respectively, is attributable to the adoption of ASU No. 2016-02, Leases (Topic 842). As of December 31, 2019, $169.1 million and $187.4 million, relating to lease ROU assets and lease liabilities, respectively, is attributable to the adoption of ASU No. 2016-02, Leases (Topic 842). See Note 27—“Leases”, in addition to the information under the heading “Recently Adopted Accounting Pronouncements” included in Note 1—“Organization and Basis of Presentation” for additional information.

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