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Related Party Transactions - Additional Information (Detail) |  |  |  |  |  |  |  |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |
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Mar. 20, 2020 USD ($) $ / shares shares | Mar. 02, 2020 shares | Sep. 27, 2019 USD ($) | Mar. 27, 2019 $ / shares shares | Mar. 05, 2019 $ / shares shares | Nov. 30, 2018 shares | Nov. 23, 2018 USD ($) shares | Mar. 09, 2018 | Dec. 13, 2017 | Oct. 25, 2016 shares | May 27, 2016 USD ($) | Jun. 05, 2015 shares | May 31, 2020 USD ($) sqrft | Jun. 30, 2020 USD ($) shares | Jun. 30, 2019 USD ($) shares | Jun. 30, 2020 USD ($) $ / shares shares | Jun. 30, 2019 USD ($) $ / shares shares | Dec. 31, 2015 USD ($) | Jun. 11, 2020 USD ($) | Feb. 05, 2020 USD ($) | Dec. 31, 2019 USD ($) shares | Feb. 27, 2019 USD ($) | Aug. 06, 2018 USD ($) | Mar. 19, 2018 USD ($) | Nov. 04, 2016 | Aug. 31, 2013 USD ($)
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commissions |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 382,640,000 | $ 422,974,000 | $ 838,495,000 | $ 853,156,000 |  |  |  |  |  |  |  |  |
Common stock, issued during the period, additional consideration |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest expense from affiliate |  |  |  |  |  |  |  |  |  |  |  |  |  | 400,000 | 0 | 400,000 | $ 0 |  |  |  |  |  |  |  |  |
Payables to Cantor related to open derivative contracts |  |  |  |  |  |  |  |  |  |  |  |  |  | 29,960,000 |  | 29,960,000 |  |  |  |  | $ 72,497,000 |  |  |  |  |
Employee loans |  |  |  |  |  |  |  |  |  |  |  |  |  | 370,770,000 |  | $ 370,770,000 |  |  |  |  | 315,590,000 |  |  |  |  |
Securities loaned |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 13,902,000 |  |  |  |  |
Limited Partnership Units Exchange Ratio |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |
Repurchase of Class A common stock, shares | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 233,172 |  |  |  |  |  |  |  |  |
Equity investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 613,000 | $ 735,000 |  |  |  |  |  |  |  |  |
Principal sum of loan due from related parties |  |  |  |  |  |  |  |  |  |  |  |  |  | 12,255,000 |  | 12,255,000 |  |  |  |  | $ 14,273,000 |  |  |  |  |
Aqua [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in authorization amount for loans and investments, other credit support arrangements |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,000,000.0 |  |  |  |  |  |
Authorization amount for loans and investments, other credit support arrangements |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 19,200,000 |  |  |  |  |  |
Equity investment |  |  |  |  |  |  |  |  |  |  |  |  |  | 400,000 | $ 400,000 | 600,000 |  |  |  |  |  |  |  |  |  |
Principal sum of loan due from related parties |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 980,000.0 |  | $ 980,000.0 |  |  |  |  |  |  |  |  |  |
Maturity date of subordinated loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Sep. 01, 2021 |  |  |  |  |  |  |  |  |  |
Loan Receivable, Description of Variable Rate Basis |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | three month LIBOR plus 600 basis points |  |  |  |  |  |  |  |  |  |
Loan Receivable, Basis Spread on Variable Rate |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.00% |  | 6.00% |  |  |  |  |  |  |  |  |  |
Stephen M. Merkel [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchange rights grant | shares |  | 360,065 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable LPU | shares |  | 360,065 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares repurchased at average price of shares | $ / shares | $ 4.0024 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed, value | $ 741,644 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of PLPU share issued for non-exchangeable | shares | 265,568 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment of withholding tax rate for common stock issue | $ 1,507,285 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable units redeemed | shares | 122,579 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related tax for redeemed PLPU | $ 661,303 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Securities [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchased senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000,000.0 |  | $ 50,000,000.0 |  |  |  |  |  |  |  |  |  |
5.125% Senior Notes Due 2021 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal amount of debt component |  |  |  |  |  |  |  |  |  |  | $ 300,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  |  | 5.125% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
3.750% Senior Notes Due October 1, 2024 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal amount of debt component |  |  | $ 300,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  | $ 36,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  | 3.75% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment percentage of the gross proceeds from the sale of shares |  |  |  |  |  |  |  | 2.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum [Member] | Debt Securities [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchased senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000,000.0 |  |  |  |  |  |  |
BGC Class B Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of Distributions |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Newmark [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares, held by parent | shares |  |  |  |  |  | 15,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class A Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 363,848,000 |  | 363,848,000 |  |  |  |  | 358,440,000 |  |  |  |  |
Issuance of Class A common stock (net of costs), shares | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 176,920 | 46,785 | 248,583 | 108,427 |  |  |  |  |  |  |  |  |
Repurchase of Class A common stock, shares | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 0 | 233,000 |  |  |  |  |  |  |  |  |
Average price per share of common stock repurchased | $ / shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5.30 |  |  |  |  |  |  |  |  |
Class A Common Stock [Member] | Stephen M. Merkel [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable LPU exchanged for issuance of shares | shares |  | 360,065 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed | shares | 185,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of shares repurchased on exchangeable LPU | 1.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares to be repurchased | shares |  |  |  | 250,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchase of Class A common stock, shares | shares |  |  |  | 233,172 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Average price per share of common stock repurchased | $ / shares |  |  |  | $ 5.30 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class A Common Stock [Member] | Mr. Lutnick [Member] | Retirement Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of Class A common stock (net of costs), shares | shares |  |  |  |  | 8,980 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Price | $ / shares |  |  |  |  | $ 6.24 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class A Common Stock [Member] | Maximum [Member] | Mr. Lutnick [Member] | Retirement Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of common stock to be issued |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 56,038 |  |  |  |
Class A Common Stock [Member] | Newmark [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares, right to exchange from Class A to Class B common stock | shares |  |  |  |  |  | 9,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class A Common Stock [Member] | Newmark OpCo [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares, held by parent | shares |  |  |  |  |  | 7,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares, right to exchange from Class A to Class B common stock | shares |  |  |  |  |  | 6,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class B Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 45,884,000 |  | 45,884,000 |  |  |  |  | 45,884,000 |  |  |  |  |
Issuance of Class A common stock (net of costs), shares | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 0 | 0 |  |  |  |  |  |  |  |  |
Class B Common Stock [Member] | Newmark [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares, right to exchange from Class A to Class B common stock | shares |  |  |  |  |  | 5,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class B Units [Member] | Lucera [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of units | shares |  |  |  |  |  |  |  |  |  | 9,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of outstanding common stock owned by parent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |
Newmark [Member] | LPUs [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate exchangeable limited partnership units purchased | shares |  |  |  |  |  | 1,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Newmark [Member] | Class A Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Spin-off received on per share |  |  |  |  |  | 0.463895 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate distribution of shares | shares |  |  |  |  |  | 131,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued | shares |  |  |  |  |  | 131,886,409 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Newmark [Member] | Class B Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Spin-off received on per share |  |  |  |  |  | 0.463895 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate distribution of shares | shares |  |  |  |  |  | 21,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of Distributions |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued | shares |  |  |  |  |  | 21,285,537 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange ratio |  |  |  |  |  | 0.9366 |  |  |  |  |  |  |  |  |  | 0.9366 |  |  |  |  |  |  |  |  |  |
Tower Bridge International Services LP [Member] | BGC Partners, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Company ownership of Tower Bridge |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 52.00% |  |  |  |  |  |  |  |  |  |
Tower Bridge International Services LP [Member] | Cantor [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling interest |  |  |  |  |  |  |  |  |  |  |  |  |  | 48.00% |  | 48.00% |  |  |  |  |  |  |  |  |  |
Aqua [Member] | BGC Partners, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of ownership interest |  |  |  |  |  |  |  |  |  |  |  |  |  | 49.00% |  | 49.00% |  |  |  |  |  |  |  |  |  |
Equity investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 700,000 |  |  |  |  |  |  |  |  |
Aqua [Member] | Cantor [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment ownership percentage |  |  |  |  |  |  |  |  |  |  |  |  |  | 51.00% |  | 51.00% |  |  |  |  |  |  |  |  |  |
Cantor [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mark-up cost of services |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.50% |  |  |  |  |  |  |  |  |  |
Cantor's share of net profit in Tower Bridge |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 400,000 | $ 600,000 | $ 200,000 | 300,000 |  |  |  |  |  |  |  |  |
Recognized related party revenues |  |  |  |  |  |  |  |  |  |  |  |  |  | 6,600,000 | 7,200,000 | 12,100,000 | 13,000,000.0 |  |  |  |  |  |  |  |  |
Service charges |  |  |  |  |  |  |  |  |  |  |  |  |  | 15,200,000 | 14,900,000 | 31,100,000 | 27,800,000 |  |  |  |  |  |  |  |  |
Compensation to leased employees |  |  |  |  |  |  |  |  |  |  |  |  |  | 10,100,000 | 8,400,000 | 20,600,000 | 18,400,000 |  |  |  |  |  |  |  |  |
Repurchase agreements |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  | 0 |  |  |  |  | $ 0 |  |  |  |  |
Recognition of share of FX gains (losses) |  |  |  |  |  |  |  |  |  |  |  |  |  | (800,000) | 700,000 | (200,000) | 900,000 |  |  |  |  |  |  |  |  |
Commissions |  |  |  |  |  |  |  |  |  |  |  |  |  | 9,000 | 69,000 | 84,000 | 147,000 |  |  |  |  |  |  |  |  |
Investment in asset-backed commercial paper program |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  | 0 |  |  |  |  | 0 |  |  |  |  |
Investment authorization in asset-backed commercial paper program |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 350,000,000.0
Reverse purchase agreement outstanding |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  | $ 0 |  |  |  |  | 0 |  |  |  |  |
Founding partner units remaining | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,129,969 |  | 2,129,969 |  |  |  |  |  |  |  |  |  |
Cantor [Member] | Lucera [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recognized related party revenues |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 200,000 | 200,000 | $ 300,000 | 300,000 |  |  |  |  |  |  |  |  |
Cantor [Member] | Revolving Credit Facility [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal amount of debt component |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 400,000,000.0 | $ 250,000,000.0 |  |
Credit agreement, description |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | The BGC Credit Agreement will mature on the earlier to occur of (a) March 19, 2021, 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% |  |  |  |  |  |  |  |  |  |
Line of credit facility, outstanding amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  | $ 0 |  |  |  |  | 0 |  |  |  |  |
Interest income from affiliate |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  | $ 0 |  |  |  |  |  |  |  |  |
Cantor [Member] | Newmark [Member] | Class A Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued | shares |  |  |  |  |  | 400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
CF & Co [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment percentage of the gross proceeds from the sale of shares |  |  |  |  |  |  |  |  | 5.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of Class A common stock (net of costs), shares | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 200,000 | 900,000 |  |  |  |  |  |  |  |  |
Aggregate proceeds from issuance of common stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 700,000 | $ 5,300,000 |  |  |  |  |  |  |  |  |
Weighted-average price per share | $ / shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4.04 | $ 6.17 |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 7,000 | $ 100,000 |  |  |  |  |  |  |  |  |
Securities loaned |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  | 0 |  |  |  |  | $ 13,900,000 |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.45% |  |  |  |  |
Guarantee fees |  |  |  |  |  |  |  |  |  |  |  |  |  | 31,000 | $ 31,000 | 63,000 | 63,000 |  |  |  |  |  |  |  |  |
CF & Co [Member] | 5.125% Senior Notes Due 2021 [Member] | Underwriting Fees [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  | $ 500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchased senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 15,000,000.0 |  | $ 15,000,000.0 |  |  |  |  |  |  |  |  |  |
CF & Co [Member] | 3.750% Senior Notes Due October 1, 2024 [Member] | Underwriting Fees [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  | $ 200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
CF & Co [Member] | Securities Financing Transaction, Fair Value [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Securities loaned |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 13,900,000 |  |  |  |  |
CF & Co [Member] | Class A Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of Class A common stock (net of costs), shares | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |
CF Group Management, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares, right to exchange from Class A to Class B common stock | shares |  |  |  |  |  |  | 712,907 |  |  |  |  | 34,649,693 |  |  |  | 23,600,000 |  |  |  |  |  |  |  |  |  |
Cantor Rights to Purchase Exchangeable Units [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares, right to exchange from Class A to Class B common stock | shares |  |  |  |  |  |  | 10,323,366 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cantor Rights to Purchase Exchangeable Units [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares, right to exchange from Class A to Class B common stock | shares |  |  |  |  |  |  |  |  |  |  |  | 34,649,693 |  |  |  | 23,600,000 |  |  |  |  |  |  |  |  |  |
Cantor Rights to Purchase Exchangeable Units [Member] | Class B Common Stock [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares, right to exchange from Class A to Class B common stock | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 23,613,420 |  |  |  |  |  |  |  |  |  |
Cantor and CFGM [Member] | Class B Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares, right to exchange from Class A to Class B common stock | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 23,613,420 |  |  |  |  |  |  |  |  |  |
Cantor and CFGM [Member] | Newmark [Member] | Class A Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued | shares |  |  |  |  |  | 449,917 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cantor and Freedom [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Receivables from Freedom |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,900,000 |  | $ 1,900,000 |  |  |  |  | 1,300,000 |  |  |  |  |
Receivables from Cantor related to open derivative contracts |  |  |  |  |  |  |  |  |  |  |  |  |  | 600,000 |  | 600,000 |  |  |  |  | 1,300,000 |  |  |  |  |
Payables to Cantor related to open derivative contracts |  |  |  |  |  |  |  |  |  |  |  |  |  | 200,000 |  | 200,000 |  |  |  |  | 2,000,000.0 |  |  |  |  |
Payables to Cantor related to pending equity trades |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,000,000.0 |  | 2,000,000.0 |  |  |  |  | 2,300,000 |  |  |  |  |
Employee Loans [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Employee loans |  |  |  |  |  |  |  |  |  |  |  |  |  | 370,800,000 |  | 370,800,000 |  |  |  |  | 315,600,000 |  |  |  |  |
Compensation expense related to employee loans |  |  |  |  |  |  |  |  |  |  |  |  |  | 17,100,000 | 7,400,000 | 31,600,000 | 14,000,000.0 |  |  |  |  |  |  |  |  |
Interest income |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,200,000 | $ 1,000,000.0 | 2,400,000 | $ 2,000,000.0 |  |  |  |  |  |  |  |  |
Castle Oak | 5.125% Senior Notes Due 2021 [Member] | Underwriting Fees [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  | $ 18,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cantor Fitzgerald Relief Fund [Member] | Accounts Payable and Accrued Liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Charitable contributions |  |  |  |  |  |  |  |  |  |  |  |  |  | 4,500,000 |  | 4,500,000 |  |  |  |  | $ 4,000,000.0 |  |  |  |  |
Cantor Fitzgerald Relief Fund [Member] | Other Expenses [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Charitable contributions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 40,000,000.0 |  |  |  |  |  |  |  |
RKF Retail Holdings LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lessee operating lease sublease term |  |  |  |  |  |  |  |  |  |  |  |  | 1 year |  |  |  |  |  |  |  |  |  |  |  |  |
Area of sublease | sqrft |  |  |  |  |  |  |  |  |  |  |  |  | 21,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Lessee operating sublease description |  |  |  |  |  |  |  |  |  |  |  |  | In May 2020, BGC U.S. OpCo entered into an arrangement to sublease excess space from RKF Retail Holdings LLC, a subsidiary of Newmark, which sublease was approved by the Audit Committee. The deal is a one-year sublease of approximately 21,000 rentable square feet in New York City. |  |  |  |  |  |  |  |  |  |  |  |  |
BGC U.S OpCo [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rent payment under lease term |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,100,000 | $ 100,000 |  | $ 100,000 |  |  |  |  |  |  |  |  |  |

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- Definition Amount which the Company is authorized to invest in an asset-backed commercial paper program with a related party. + References No definition available. + Details
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- Definition Guarantee fee expenses. + References No definition available. + Details
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