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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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Jun. 28, 2021 USD ($) $ / shares shares | Jun. 24, 2021 | Apr. 29, 2021 USD ($) shares | Apr. 23, 2021 USD ($) $ / shares shares | Mar. 31, 2021 USD ($) shares | Mar. 08, 2021 USD ($) | Feb. 22, 2021 USD ($) shares | Aug. 14, 2020 USD ($) | 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 shares | Sep. 27, 2019 USD ($) | Nov. 23, 2018 USD ($) shares | Aug. 06, 2018 USD ($) | Jul. 24, 2018 USD ($) | Oct. 25, 2016 shares | May 27, 2016 USD ($) | Jun. 05, 2015 shares | May 31, 2020 USD ($) ft² | Jun. 30, 2021 USD ($) shares | Jun. 30, 2020 USD ($) shares | Jun. 30, 2021 USD ($) shares | Jun. 30, 2020 USD ($) $ / shares shares | Dec. 31, 2015 USD ($) | Apr. 08, 2021 shares | Feb. 25, 2021 USD ($) | Dec. 31, 2020 USD ($) | Jun. 11, 2020 USD ($) | Feb. 05, 2020 USD ($) | Nov. 28, 2018 | Mar. 19, 2018 USD ($) | Nov. 04, 2016 | Aug. 31, 2013 USD ($)
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commissions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 389,768,000 | $ 382,640,000 | $ 824,988,000 | $ 838,495,000 |  |  |  |  |  |  |  |  |  |
Payables to Cantor related to open derivative contracts |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 79,920,000 |  | 79,920,000 |  |  |  |  | $ 36,252,000 |  |  |  |  |  |
Employee loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 370,800,000 |  | 370,800,000 |  |  |  |  | 408,142,000 |  |  |  |  |  |
Number of H units redeemed (in shares) | shares | 1,131,774 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related to H units redeemed | $ 7,017,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of preferred H units redeemed (in shares) | shares | 1,018,390 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related tax for redeemed preferred H units | $ 7,983,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 625,000 | 613,000 |  |  |  |  |  |  |  |  |  |
Receivables from related parties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7,890,000 |  | 7,890,000 |  |  |  |  | 11,953,000 |  |  |  |  |  |
Aqua |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |
Equity investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 300,000 | $ 400,000 | 600,000 | $ 600,000 |  |  |  |  |  |  |  |  |  |
Receivables from related parties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 980,000.0 |  | $ 980,000.0 |  |  |  |  |  |  |  |  |  |  |
Loan Receivable, Basis Spread on Variable Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.00% |  | 6.00% |  |  |  |  |  |  |  |  |  |  |
Stephen M. Merkel |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of PLPU share issued for non-exchangeable (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  | 265,568 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment of withholding tax rate for common stock issue |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,507,285 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed, value |  |  |  |  |  |  |  |  |  |  |  |  | $ 741,644 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable units redeemed (in shares) | shares |  |  | 108,350 |  |  |  |  |  |  |  | 142,989 |  | 122,579 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Non-exchangeable PSUs (in shares) | shares |  |  | 273,612 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment Related To Non-exchangeable PSU redeemed upon exchange |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable PPSUs redeemed | shares |  |  | 101,358 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable PPSUs (in shares) | shares |  |  | 250,659 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment Related To Non-exchangeable PPSU redeemed upon exchange |  |  | $ 575,687 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related tax for redeemed PLPU |  |  |  |  |  |  |  |  |  |  | $ 846,182 |  | $ 661,303 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares repurchased at average price of shares (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  |  |  |  | $ 4.0024 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mr. Lutnick | Exchange LPUs For Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable LPU exchanged for issuance of shares (in shares) | shares | 520,380 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange share price (in dollars per share) | $ / shares | $ 5.86 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mr. Lutnick | Exchange PLPUs for Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable LPU (in shares) | shares | 88,636 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable PLPUs (in shares) | shares | 425,765 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related tax for redeemed PLPU | $ 1,525,705 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shaun D. Lynn |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchange rights grant (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 883,348 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of PLPU share issued for non-exchangeable (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 245,140 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment of withholding tax rate for common stock issue |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,099,599 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable units redeemed (in shares) | shares |  |  |  |  |  |  |  |  |  |  | 221,239 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable PLPUs (in shares) | shares |  |  |  |  |  |  |  |  |  |  | 23,901 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related tax for redeemed PLPU |  |  |  |  |  |  |  |  |  |  | $ 992,388 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of remaining exchangeable LPUs (in shares) | shares |  |  |  |  |  |  |  |  |  |  | 86,126 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related to exchangeable LPUs and PLPUs redeemed upon exchange in connection with LLP status |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sean A. Windeatt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchange rights grant (in shares) | shares |  |  |  |  |  |  | 123,713 |  |  |  |  |  |  | 519,725 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable LPU (in shares) | shares |  |  |  |  |  |  | 123,713 |  |  |  |  |  |  | 519,725 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of PLPU share issued for non-exchangeable (in shares) | shares |  |  |  |  |  |  | 28,477 |  |  |  |  |  |  | 97,656 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment of withholding tax rate for common stock issue |  |  |  |  |  |  | $ 178,266 |  |  |  |  |  |  | $ 645,779 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares authorized to be repurchased (in shares) | shares |  |  |  | 123,713 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange share price (in dollars per share) | $ / shares |  |  |  | $ 5.65 |  |  |  |  |  |  |  |  |  | $ 5.38 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed (in shares) | shares |  |  |  | 28,477 |  |  |  |  |  | 20,849 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed, value |  |  |  | $ 178,266 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Remaining number of shares authorized to be repurchased (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 62,211 |  |  |  |  |  |  |  |
Number of non-exchangeable units redeemed (in shares) | shares |  |  |  |  |  |  |  |  |  | 96,216 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable PLPUs (in shares) | shares |  |  |  |  |  |  |  |  |  | 1,440 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related tax for redeemed PLPU |  |  |  |  |  |  |  |  |  | $ 637,866 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mr. Windeatt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchange rights grant (in shares) | shares |  |  |  |  |  |  |  |  |  | 40,437 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable LPU (in shares) | shares |  |  |  |  |  |  |  |  | 43,890 | 40,437 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of PLPU share issued for non-exchangeable (in shares) | shares |  |  |  |  |  |  |  |  | 17,068 | 21,774 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed (in shares) | shares |  |  |  |  |  |  |  |  | 3,681 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable PLPUs (in shares) | shares |  |  |  |  |  |  |  |  | 1,431 | 21,774 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment related tax for redeemed PLPU |  |  |  |  |  |  |  |  | $ 194,086 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, authorized repurchase amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000,000.0 |  |  |  |  |
Debt instrument, remaining authorized repurchase amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000,000.0 |  | $ 50,000,000.0 |  |  |  |  |  |  |  |  |  |  |
5.125% Senior Notes Due 2021 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal amount of debt component |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 300,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.125% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
5.375% Senior Notes Due 2023 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal amount of debt component |  |  |  |  |  |  |  |  |  | $ 300,000,000.0 |  |  |  |  |  |  |  |  |  | $ 300,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.125% |  |  | 5.125% |  | 5.125% |  |  |  |  |  |  |  |  |  |  |
14.66% Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal amount of debt component |  |  |  |  |  |  |  | $ 44,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  | 14.66% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class B Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of Class A common stock (net of costs) (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 0 | 0 |  |  |  |  |  |  |  |  |  |
Class A Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of Class A common stock (net of costs) (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,542 | 176,920 | 265,915 | 248,583 |  |  |  |  |  |  |  |  |  |
Aggregate proceeds from issuance of common stock |  |  |  |  |  | $ 300,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchased during period (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 16,500,000 |  | 17,500,000 |  |  |  |  |  |  |  |  |  |  |
Class A Common Stock | Stephen M. Merkel |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of Class A common stock (net of costs) (in shares) | shares |  |  | 108,350 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchange rights grant (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 360,065 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable LPU (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 360,065 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable LPU exchanged for issuance of shares (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 360,065 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange share price (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  |  | $ 2.76 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed (in shares) | shares |  |  |  |  |  |  |  |  |  |  | 174,765 |  | 185,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchased during period (in shares) | shares |  |  | 108,350 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of shares repurchased on exchangeable LPU |  |  |  |  |  |  |  |  |  |  |  |  | 1.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class A Common Stock | Mr. Lutnick | Exchange LPUs For Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of Class A common stock (net of costs) (in shares) | shares | 520,380 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued net of shares for tax withholdings (in shares) | shares | 365,229 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class A Common Stock | Mr. Lutnick | Exchange PLPUs for Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of Class A common stock (net of costs) (in shares) | shares | 88,636 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued net of shares for tax withholdings (in shares) | shares | 41,464 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class A Common Stock | Shaun D. Lynn |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of non-exchangeable LPU (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 883,348 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable LPU exchanged for issuance of shares (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 883,348 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange share price (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  |  | $ 2.76 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed (in shares) | shares |  |  |  |  |  |  |  |  |  |  | 797,222 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class A Common Stock | Sean A. Windeatt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable LPU exchanged for issuance of shares (in shares) | shares |  |  |  |  |  |  | 123,713 |  |  |  |  |  |  | 519,725 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange share price (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  | $ 2.90 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed (in shares) | shares |  |  |  |  |  |  |  |  |  | 436,665 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class A Common Stock | Mr. Windeatt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable LPU exchanged for issuance of shares (in shares) | shares |  |  |  |  |  |  |  |  | 37,660 | 40,437 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange share price (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  | $ 4.16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares redeemed (in shares) | shares |  |  |  |  |  |  |  |  | 40,209 | 40,437 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of exchangeable PLPUs (in shares) | shares |  |  |  |  |  |  |  |  | 15,637 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares repurchased at average price of shares (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  | $ 2.90 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class B Units | Lucera |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of units (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of outstanding common stock owned by parent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |
Tower Bridge International Services LP | BGC Partners, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Company ownership of Tower Bridge |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 52.00% |  |  |  |  |  |  |  |  |  |  |
Tower Bridge International Services LP | Cantor |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 48.00% |  | 48.00% |  |  |  |  |  |  |  |  |  |  |
Aqua | BGC Partners, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of ownership interest |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 49.00% |  | 49.00% |  |  |  |  |  |  |  |  |  |  |
Aqua | Cantor |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment ownership percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 51.00% |  | 51.00% |  |  |  |  |  |  |  |  |  |  |
Cantor |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mark-up cost of services |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.50% |  |  |  |  |  |  |  |  |  |  |
Cantor's share of net profit (loss) in Tower Bridge |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 500,000 | $ 400,000 | $ 600,000 | $ 200,000 |  |  |  |  |  |  |  |  |  |
Recognized related party revenues |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4,200,000 | 6,600,000 | 8,000,000.0 | 12,100,000 |  |  |  |  |  |  |  |  |  |
Service charges |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 20,000,000.0 | 15,200,000 | 41,100,000 | 31,100,000 |  |  |  |  |  |  |  |  |  |
Compensation to leased employees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15,500,000 | 10,100,000 | 31,400,000 | 20,600,000 |  |  |  |  |  |  |  |  |  |
Recognition of share of FX gains |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (200,000) | (800,000) | 23,000 | (200,000) |  |  |  |  |  |  |  |  |  |
Commissions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 37,000 | 9,000 | 62,000 | 84,000 |  |  |  |  |  |  |  |  |  |
Investment authorization in asset-backed commercial paper program |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 350,000,000.0
Investment in asset-backed commercial paper program |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  | 0 |  |  |  |  | 0 |  |  |  |  |  |
Common stock, issued during the period, additional consideration |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reverse purchase agreement outstanding |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  | 0 |  |  |  |  | 0 |  |  |  |  |  |
Cantor | Lucera |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recognized related party revenues |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100,000 | 200,000 | 200,000 | 300,000 |  |  |  |  |  |  |  |  |  |
Cantor | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal amount of debt component |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 400,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 250,000,000.0 |  |
Debt instrument, extended term |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1 year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, non-extension notice term |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6 months |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  |  |  |  |  |  |  |  |  |  |  |  | 1.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit facility, outstanding amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  | 0 |  |  |  |  | 0 |  |  |  |  |  |
Interest expense from affiliate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 | 400,000 | $ 0 | 400,000 |  |  |  |  |  |  |  |  |  |
CF Group Management, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Right to exchange from Class A to Class B common stock (up to) (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 700,000 |  |  |  |  | 34,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, issued during the period, additional consideration |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cantor Rights to Purchase Exchangeable Units |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Right to exchange from Class A to Class B common stock (up to) (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10,300,000 |  |  |  |  |  |  |  |  | 23,600,000 |  |  |  |  |  |  |  |  |  |  |
Limited Partnership Units Exchange Ratio |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |  |
Cantor Rights to Purchase Exchangeable Units | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Right to exchange from Class A to Class B common stock (up to) (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 34,600,000 |  |  |  | 23,600,000 |  |  |  |  |  |  |  |  |  |  |
Cantor Rights to Purchase Exchangeable Units | BGC Holdings LPUs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Limited Partnership Units Exchange Ratio |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate cantor units as result of redemption | shares |  |  |  |  | 1,149,684 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate consideration of cantor units as result of redemption |  |  |  |  | $ 2,104,433 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of founding partner interests as result of redemption (in shares) | shares |  |  |  |  | 1,149,684 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate cantor units (in shares) | shares |  |  |  |  | 1,618,376 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate consideration of cantor units |  |  |  |  | $ 3,040,411 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of founding partner interests exchanged (in shares) | shares |  |  |  |  | 1,618,376 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Founding partner units remaining (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,600,000 |  | 1,600,000 |  |  |  |  |  |  |  |  |  |  |
Cantor and CFGM | Conversion of Class B Common Stock to Class A Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, conversion ratio |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |  |  | 1 |  |  |
Cantor and CFGM | Class B Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Right to exchange from Class A to Class B common stock (up to) (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 23,600,000 |  |  |  |  |  |  |  |  |  |  |
Cantor and Freedom |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Receivables from Freedom |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,100,000 |  | $ 2,100,000 |  |  |  |  | 1,400,000 |  |  |  |  |  |
Receivables from Cantor related to open derivative contracts |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4,600,000 |  | 4,600,000 |  |  |  |  | 600,000 |  |  |  |  |  |
Payables to Cantor related to open derivative contracts |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4,500,000 |  | 4,500,000 |  |  |  |  | 100,000 |  |  |  |  |  |
Payables to Cantor related to pending equity trades |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 20,100,000 |  | 20,100,000 |  |  |  |  | 26,000,000.0 |  |  |  |  |  |
Employee Loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Employee loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 370,800,000 |  | 370,800,000 |  |  |  |  | 408,100,000 |  |  |  |  |  |
Compensation expense related to employee loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 19,100,000 | 17,100,000 | 34,700,000 | 31,600,000 |  |  |  |  |  |  |  |  |  |
Interest income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,300,000 | $ 1,700,000 | 5,500,000 | $ 4,400,000 |  |  |  |  |  |  |  |  |  |
Employee Loans | Disposal Group, Held-for-sale, Not Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Employee loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 14,500,000 |  | $ 14,500,000 |  |  |  |  |  |  |  |  |  |  |
CF & Co |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of Class A common stock (net of costs) (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 0 | 200,000 |  |  |  |  |  |  |  |  |  |
Aggregate proceeds from issuance of common stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 700,000 |  |  |  |  |  |  |  |  |  |
Weighted-average price (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4.04 |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  | $ 0 | $ 7,000 |  |  |  |  |  |  |  |  |  |
Securities loaned |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  | 0 |  |  |  |  | 0 |  |  |  |  |  |
Guarantee fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 31,000 | $ 31,000 | 63,000 | 63,000 |  |  |  |  |  |  |  |  |  |
CF & Co | Underwriting Fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  | $ 14,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
CF & Co | 5.125% Senior Notes Due 2021 | Underwriting Fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchased senior notes |  |  |  |  |  |  |  | $ 15,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
CF & Co | 5.375% Senior Notes Due 2023 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Advisory fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
CF & Co | 5.375% Senior Notes Due 2023 | Underwriting Fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
CF & Co | 4.375% Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchased senior notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 14,500,000 |  | 14,500,000 |  |  |  |  |  |  |  |  |  |  |
CF & Co | 4.375% Senior Notes | Underwriting Fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  |  | $ 200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Castle Oak | 5.125% Senior Notes Due 2021 | Underwriting Fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 18,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Castle Oak | 5.375% Senior Notes Due 2023 | Underwriting Fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 41,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Castle Oak | 3.750% Senior Notes Due October 1, 2024 | Underwriting Fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cantor Fitzgerald Relief Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional expense associated with charitable contributions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,100,000 |  | 1,100,000 |  |  |  |  | 1,100,000 |  |  |  |  |  |
Cantor Fitzgerald Relief Fund | Accounts Payable and Accrued Liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Remaining liability associated with charitable contributions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,500,000 |  | 1,500,000 |  |  |  |  | $ 1,600,000 |  |  |  |  |  |
Cantor Fitzgerald Relief Fund | Other Expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Charitable contributions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 40,000,000.0 |  |  |  |  |  |  |  |  |
RKF Retail Holdings LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lessee operating lease sublease term |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1 year |  |  |  |  |  |  |  |  |  |  |  |  |  |
Area of sublease | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 21,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
BGC U.S OpCo |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating lease cost |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rent payment under lease term |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 100,000 | $ 100,000 | $ 400,000 | $ 100,000 |  |  |  |  |  |  |  |  |  |
Aurel BGC SAS | Cantor |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transactions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of revenue shared between related parties |  | 80.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue sharing agreement, term |  | 12 months |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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