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Related Party Transactions - Additional Information (Detail) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |
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Mar. 08, 2021 USD ($) | Aug. 14, 2020 USD ($) | Aug. 06, 2020 USD ($) $ / shares shares | Aug. 05, 2020 USD ($) $ / shares shares | Jul. 30, 2020 USD ($) $ / shares shares | Jul. 10, 2020 USD ($) | Mar. 20, 2020 USD ($) $ / shares shares | Mar. 02, 2020 USD ($) shares | Sep. 27, 2019 USD ($) | Nov. 23, 2018 USD ($) shares | Jul. 24, 2018 USD ($) | Oct. 25, 2016 shares | May 27, 2016 USD ($) | Jun. 05, 2015 shares | May 31, 2020 USD ($) sqrft | Mar. 31, 2021 USD ($) shares | Mar. 31, 2020 USD ($) $ / shares shares | Sep. 30, 2020 USD ($) | Dec. 31, 2015 USD ($) | Feb. 25, 2021 USD ($) | Dec. 31, 2020 USD ($) | Jun. 11, 2020 USD ($) | Feb. 05, 2020 USD ($) | Aug. 06, 2018 USD ($) | Mar. 19, 2018 USD ($) | Nov. 04, 2016 | Aug. 31, 2013 USD ($)
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchase agreements |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 94,000 |  |  |  |  |  |  |  |  |  |  |
Commissions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 435,220,000 | $ 455,855,000 |  |  |  |  |  |  |  |  |  |
Common stock, issued during the period, additional consideration |  |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest expense from affiliate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 |  |  |  |  |  |  |  |  |  |
Payables to Cantor related to open derivative contracts |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 47,501,000 |  |  |  |  | $ 36,252,000 |  |  |  |  |  |
Employee loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 419,638,000 |  |  |  |  | 408,142,000 |  |  |  |  |  |
Limited Partnership Units Exchange Ratio |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |  |
Equity investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 355,000 | 245,000 |  |  |  |  |  |  |  |  |  |
Receivables from related parties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6,919,000 |  |  |  |  | $ 11,953,000 |  |  |  |  |  |
Aqua [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in authorization amount for loans and investments, other credit support arrangements |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,000,000.0 |  |  | $ 2,000,000.0 |  |  |  |
Authorization amount for loans and investments, other credit support arrangements |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 20,200,000 |  |  | $ 20,200,000 |  |  |  |
Equity investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400,000 | $ 200,000 |  |  |  |  |  |  |  |  |  |
Receivables from related parties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 980,000.0 |  |  |  |  |  |  |  |  |  |  |
Maturity date of subordinated loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Sep. 01, 2022 |  |  |  |  |  |  |  |  |  |  |
Loan Receivable, Description of Variable Rate Basis |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | three month LIBOR plus 600 basis points |  |  |  |  |  |  |  |  |  |  |
Loan Receivable, Basis Spread on Variable Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 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 |  |  |  |  | 142,989 |  | 122,579 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related tax for redeemed PLPU |  |  |  |  | $ 846,182 |  | $ 661,303 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shaun D. Lynn [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchange rights grant | shares |  |  |  |  |  |  |  | 883,348 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable LPU | shares |  |  |  |  |  |  |  | 883,348 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of PLPU share issued for non-exchangeable | shares |  |  |  |  |  |  |  | 245,140 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment of withholding tax rate for common stock issue |  |  |  |  |  |  |  | $ 1,099,599 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable units redeemed | shares |  |  |  |  | 221,239 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related tax for redeemed PLPU |  |  |  |  | $ 992,388 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of remaining exchangeable LPUs | shares |  |  |  |  | 86,126 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable PLPUs | shares |  |  |  |  | 23,901 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related to exchangeable LPUs and PLPUs redeemed upon exchange in connection with LLP status |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sean A. Windeatt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchange rights grant | shares |  |  |  |  |  |  |  | 519,725 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable LPU | shares |  |  |  |  |  |  |  | 519,725 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed | shares |  |  |  | 20,849 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of PLPU share issued for non-exchangeable | shares |  |  |  |  |  |  |  | 97,656 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment of withholding tax rate for common stock issue |  |  |  |  |  |  |  | $ 645,779 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable units redeemed | shares |  |  |  | 96,216 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related tax for redeemed PLPU |  |  |  | $ 637,866 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable PLPUs | shares |  |  |  | 1,440 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related to exchangeable LPUs and PLPUs redeemed upon exchange in connection with LLP status |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mr. Windeatt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchange rights grant | shares |  |  |  | 40,437 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable LPU | shares |  |  | 43,890 | 40,437 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed | shares |  |  | 3,681 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of PLPU share issued for non-exchangeable | shares |  |  | 17,068 | 21,774 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related tax for redeemed PLPU |  |  | $ 194,086 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable PLPUs | shares |  |  | 1,431 | 21,774 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related to exchangeable LPUs and PLPUs redeemed upon exchange in connection with LLP status |  |  | $ 0 | $ 136,305 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Transaction charge of grant of exchangeability |  |  | $ 381,961 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Securities [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchased senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 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% |  |  |  |  |  |  |  |  |  |  |  |  |  |
5.375% Senior Notes Due 2023 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal amount of debt component |  |  |  |  |  |  |  |  |  |  | $ 450,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  |  | 5.375% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
4.375% Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal amount of debt component |  |  |  |  |  | $ 300,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 36,000 |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  | 4.375% |  |  |  |  |  |  |  |  |  | 4.375% |  |  |  |  | 4.375% |  |  |  |  |  |
5.125% Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal amount of debt component |  |  |  |  |  |  |  |  |  |  |  |  | $ 300,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  | 5.125% |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.125% |  |  |  |  | 5.125% |  |  |  |  |  |
14.66% Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal amount of debt component |  | $ 44,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  | 14.66% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class B Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of Class A common stock (net of costs), shares | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 |  |  |  |  |  |  |  |  |  |
Class A Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of Class A common stock (net of costs), shares | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 262,373 | 71,663 |  |  |  |  |  |  |  |  |  |
Aggregate proceeds from issuance of common stock | $ 300,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  | 174,765 |  | 185,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of shares repurchased on exchangeable LPU |  |  |  |  |  |  | 1.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange share price | $ / shares |  |  |  |  | $ 2.76 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class A Common Stock [Member] | Shaun D. Lynn [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable LPU exchanged for issuance of shares | shares |  |  |  |  |  |  |  | 883,348 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed | shares |  |  |  |  | 797,222 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange share price | $ / shares |  |  |  |  | $ 2.76 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class A Common Stock [Member] | Sean A. Windeatt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable LPU exchanged for issuance of shares | shares |  |  |  |  |  |  |  | 519,725 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed | shares |  |  |  | 436,665 |  |  |  | 436,665 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange share price | $ / shares |  |  |  | $ 2.90 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class A Common Stock [Member] | Mr. Windeatt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable LPU exchanged for issuance of shares | shares |  |  | 37,660 | 40,437 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed | shares |  |  | 40,209 | 40,437 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares repurchased at average price of shares | $ / shares |  |  |  | $ 2.90 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange share price | $ / shares |  |  | $ 4.16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable PLPUs | shares |  |  | 15,637 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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% |
Maximum [Member] | Debt Securities [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchased senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000,000.0 |  |  |  |  |
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% |  |  |  |  |  |  |  |  |  |  |
Aqua [Member] | BGC Partners, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of ownership interest |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 49.00% |  |  |  |  |  |  |  |  |  |  |
Aqua [Member] | Cantor [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment ownership percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 51.00% |  |  |  |  |  |  |  |  |  |  |
Cantor [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mark-up cost of services |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.50% |  |  |  |  |  |  |  |  |  |  |
Cantor's share of net profit (loss) in Tower Bridge |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 40,000 | $ (134,000) |  |  |  |  |  |  |  |  |  |
Recognized related party revenues |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,800,000 | 5,500,000 |  |  |  |  |  |  |  |  |  |
Service charges |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 21,100,000 | 15,900,000 |  |  |  |  |  |  |  |  |  |
Compensation to leased employees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15,800,000 | 10,400,000 |  |  |  |  |  |  |  |  |  |
Repurchase agreements |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100,000 |  |  |  |  | $ 0 |  |  |  |  |  |
Recognition of share of FX gains |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100,000 | 1,700,000 |  |  |  |  |  |  |  |  |  |
Commissions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25,000 | 75,000 |  |  |  |  |  |  |  |  |  |
Investment in asset-backed commercial paper program |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 |  |  |  |  |  |  |  |  |  |
Investment authorization in asset-backed commercial paper program |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 350,000,000.0
Reverse purchase agreement outstanding |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  | 0 |  |  |  |  |  |
Founding partner units remaining | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 500,000 |  |  |  |  |  |  |  |  |  |  |
Cantor [Member] | Lucera [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recognized related party revenues |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 100,000 | 100,000 |  |  |  |  |  |  |  |  |  |
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, 2022, 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 |  |  |  |  |  |
CF Group Management, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares, right to exchange from Class A to Class B common stock | shares |  |  |  |  |  |  |  |  |  | 700,000 |  |  |  | 34,600,000 |  | 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 |  |  |  |  |  |  |  |  |  | 10,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  | 34,600,000 |  | 23,600,000 |  |  |  |  |  |  |  |  |  |  |
Cantor Rights to Purchase Exchangeable Units [Member] | Maximum [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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 23,600,000 |  |  |  |  |  |  |  |  |  |  |
Cantor Rights to Purchase Exchangeable Units [Member] | BGC Holdings LPUs [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate cantor units as result of redemption | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,149,684 |  |  |  |  |  |  |  |  |  |  |
Aggregate consideration of cantor units as result of redemption |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,104,433,000 |  |  |  |  |  |  |  |  |  |  |
Number of founding partner interests as result of redemption | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,149,684 |  |  |  |  |  |  |  |  |  |  |
Aggregate cantor units | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,618,376 |  |  |  |  |  |  |  |  |  |  |
Aggregate consideration of cantor units |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,040,411,000 |  |  |  |  |  |  |  |  |  |  |
Number of founding partner interests exchanged | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,618,376 |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 23,600,000 |  |  |  |  |  |  |  |  |  |  |
Cantor and Freedom [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Receivables from Freedom |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,200,000 |  |  |  |  | 1,400,000 |  |  |  |  |  |
Receivables from Cantor related to open derivative contracts |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,800,000 |  |  |  |  | 600,000 |  |  |  |  |  |
Payables to Cantor related to open derivative contracts |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,500,000 |  |  |  |  | 100,000 |  |  |  |  |  |
Payables to Cantor related to pending equity trades |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 11,000,000.0 |  |  |  |  | 26,000,000.0 |  |  |  |  |  |
Employee Loans [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Employee loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 419,600,000 |  |  |  |  | 408,100,000 |  |  |  |  |  |
Compensation expense related to employee loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15,600,000 | 14,600,000 |  |  |  |  |  |  |  |  |  |
Interest income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,200,000 | $ 2,700,000 |  |  |  |  |  |  |  |  |  |
CF & Co [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchase agreements |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 100,000 |  |  |  |  | 0 |  |  |  |  |  |
Issuance of Class A common stock (net of costs), shares | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 200,000 |  |  |  |  |  |  |  |  |  |
Aggregate proceeds from issuance of common stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 700,000 |  |  |  |  |  |  |  |  |  |
Weighted-average price per share | $ / shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4.04 |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 7,000 |  |  |  |  |  |  |  |  |  |
Securities loaned |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  | 0 |  |  |  |  |  |
Guarantee fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 31,000 | $ 31,000 |  |  |  |  |  |  |  |  |  |
CF & Co [Member] | Underwriting Fees [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  | $ 14,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |
CF & Co [Member] | 5.375% Senior Notes Due 2023 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Advisory fees |  |  |  |  |  |  |  |  |  |  | $ 200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
CF & Co [Member] | 5.375% Senior Notes Due 2023 [Member] | Underwriting Fees [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  | 300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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] | 4.375% Senior Notes [Member] | Underwriting Fees [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  | $ 200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchased senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 14,500,000 |  |  |  |  |  |  |  |  |  |  |
CF & Co [Member] | Class A Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of Class A common stock (net of costs), shares | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |
Castle Oak | 5.125% Senior Notes Due 2021 [Member] | Underwriting Fees [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  |  |  | $ 18,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Castle Oak | 5.375% Senior Notes Due 2023 [Member] | Underwriting Fees [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  | $ 41,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cantor Fitzgerald Relief Fund [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Charitable contributions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,100,000 |  |  |  |  | 1,100,000 |  |  |  |  |  |
Cantor Fitzgerald Relief Fund [Member] | Accounts Payable and Accrued Liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Charitable contributions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,600,000 |  |  |  |  | $ 1,600,000 |  |  |  |  |  |
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 | $ 300,000 |  |  |  |  |  |  |  |  |  |  |

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