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Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2017 | Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017 | Dec. 13, 2017 | Jun. 22, 2016 | Jun. 21, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Aggregate number of shares grant of future awards |  | 184,700,000 |  |  |  |  |
Number of share-equivalent limited partnership units exchangeable into shares | 20,000,000 | 22,900,000 |  | 20,000,000 |  |  |
Number of standalone limited partnership units exchangeable shares |  | 900,000 |  |  |  |  |
Notional value with post-termination pay-out amount | $ 242,100 | $ 148,700 |  | $ 242,100 |  |  |
Aggregate estimated fair value of limited partnership units | $ 45,700 | $ 35,100 |  | $ 45,700 |  |  |
Number of unvested limited partnership units with post-termination pay-out | 24,100,000 | 14,500,000 |  | 24,100,000 |  |  |
Number of outstanding limited partnership units, unvested | 13,300,000 | 6,700,000 |  | 13,300,000 |  |  |
Income allocated to limited partnership units |  | $ 9,000 | $ 9,400 |  |  |  |
Restricted stock unit, Conversion description |  | Each BGC RSU is settled in one share of BGC Class A common stock upon completion of the vesting period. |  |  |  |  |
Shares, Forfeited |  | 0 | 0 |  |  |  |
Number of shares released the restrictions |  | 500,000 | 1,000,000 |  |  |  |
Cash received per share upon conversion |  | $ 6.10 |  |  |  |  |
Deferred cash compensation expense recognized |  | $ 2,379 | $ 3,777 |  |  |  |
Liability for deferred cash compensation awards |  | 8,200 |  |  |  |  |
Deferred cash compensation expense not yet recognized |  | $ 5,300 |  |  |  |  |
Deferred compensation expense, expected period of recognition |  | 1 year 10 months 20 days |  |  |  |  |
Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Limited partnership units outstanding | 1,409,747 | 1,019,100 |  | 1,409,747 |  |  |
Number of units, granted |  | 51,976 | 100,000 |  |  |  |
Number of units, forfeited |  | (29,572) |  |  |  |  |
Equity-based compensation expense |  | $ 1,500 | $ 1,100 |  |  |  |
Aggregate estimated grant date fair values |  | 600 | 1,000 |  |  |  |
Value of shares withheld |  | 1,700 | $ 2,000 |  |  |  |
Aggregate estimated grant date fair value of outstanding RSUs | $ 12,300 | 9,200 |  | $ 12,300 |  |  |
Unrecognized compensation expense related to unvested RSUs |  | $ 9,000 |  |  |  |  |
Weighted average remaining contractual term |  | 1 year 8 months 8 days |  | 1 year 8 months 4 days |  |  |
Restricted Shares [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Limited partnership units outstanding |  | 8,700,000 |  |  |  |  |
Minimum [Member] | Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Restricted shares, restriction period |  | 2 years |  |  |  |  |
Minimum [Member] | Restricted Shares [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Restricted shares saleable period |  | 5 years |  |  |  |  |
Maximum [Member] | Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Restricted shares, restriction period |  | 4 years |  |  |  |  |
Maximum [Member] | Restricted Shares [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Restricted shares saleable period |  | 10 years |  |  |  |  |
Limited Partnership Units [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Limited partnership units outstanding | 123,631,195 | 129,306,880 |  | 123,631,195 |  |  |
Number of units, granted |  | 11,343,446 |  |  |  |  |
Number of units, redeemed/exchanged |  | (5,629,072) |  |  |  |  |
Number of units, forfeited |  | (38,689) |  |  |  |  |
Number of limited partnership units granted exchangeability |  | 4,700,000 | 5,400,000 |  |  |  |
Equity-based compensation expense |  | $ 56,200 | $ 53,800 |  |  |  |
Aggregate estimated fair value of limited partnership units | $ 20,600 | 16,700 |  | $ 20,600 |  |  |
Compensation expense (benefit) before income tax related to limited partnership units |  | $ (8,700) | 6,400 |  |  |  |
Limited Partnership Units [Member] | Minimum [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Restricted shares, restriction period |  | 3 years |  |  |  |  |
Limited Partnership Units [Member] | Maximum [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Restricted shares, restriction period |  | 5 years |  |  |  |  |
Newmark [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Number of share-equivalent limited partnership units exchangeable into shares | 9,100,000 | 10,800,000 |  | 9,100,000 |  |  |
Notional value with post-termination pay-out amount |  | $ 21,600 |  |  |  |  |
Number of unvested limited partnership units with post-termination pay-out | 11,000,000 | 6,600,000 |  | 11,000,000 |  |  |
Number of outstanding limited partnership units, unvested | 6,000,000 | 3,000,000 |  | 6,000,000 |  |  |
Newmark [Member] | Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Number of units, granted |  | 40,000 |  |  |  |  |
Aggregate estimated grant date fair values |  | $ 500 |  |  |  |  |
Aggregate estimated grant date fair value of outstanding RSUs |  | $ 500 |  |  |  |  |
Weighted average remaining contractual term |  | 2 years 5 months 4 days |  |  |  |  |
Newmark [Member] | Limited Partnership Units [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Limited partnership units outstanding | 56,200,000 | 59,800,000 |  | 56,200,000 |  |  |
Number of units, increase in period |  | 3,600,000 |  |  |  |  |
Number of units, granted |  | 6,100,000 |  |  |  |  |
Number of units, redeemed/exchanged |  | (2,500,000) |  |  |  |  |
Number of units, forfeited |  | (17,600) |  |  |  |  |
Number of limited partnership units granted exchangeability |  | 2,100,000 |  |  |  |  |
Newmark [Member] | Standalone Limited Partnership Units [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Aggregate estimated fair value of limited partnership units |  | $ 2,100 |  |  |  |  |
Number of unvested limited partnership units with post-termination pay-out |  | 1,500,000 |  |  |  |  |
Number of outstanding limited partnership units, unvested |  | 1,100,000 |  |  |  |  |
GFI Group Inc. [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Deferred cash compensation expense recognized |  | $ 1,300 | 3,800 |  |  |  |
BPF [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Deferred cash compensation expense recognized |  | 1,100 | $ 200 |  |  |  |
Liability for deferred cash compensation awards | $ 400 | $ 600 |  | $ 400 |  |  |
Class A Common Stock [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Number of share-equivalent limited partnership units exchangeable into shares | 20,000,000 | 22,900,000 |  | 20,000,000 |  |  |
Shares, Forfeited |  |  |  | 183,246 |  |  |
Class A Common Stock [Member] | Chief Executive Officer and Chief Financial Officer [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Deferred stock issued |  | 1,000,000 |  |  |  |  |
Class A Common Stock [Member] | Newmark [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Shares authorized to be delivered pursuant to awards granted |  |  |  |  | 50,000,000 |  |
Class A Common Stock [Member] | Newmark [Member] | Chief Executive Officer and Chief Financial Officer [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Deferred cash compensation expense recognized | $ 40,300 |  |  |  |  |  |
Deferred stock issued |  | 300,000 |  |  |  |  |
Class A Common Stock [Member] | Newmark [Member] | Minimum [Member] | Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Restricted shares, restriction period |  | 2 years |  |  |  |  |
Class A Common Stock [Member] | Newmark [Member] | Maximum [Member] | Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Restricted shares, restriction period |  | 3 years |  |  |  |  |
Equity Plan [Member] | Class A Common Stock [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Shares authorized to be delivered pursuant to awards granted |  |  |  |  |  | 400,000,000 | 350,000,000
Newmark Long Term Incentive Plan [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Aggregate number of shares grant of future awards |  | 50,000,000 |  |  |  |  |

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