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Condensed Consolidated Statements of Changes in Equity (Parenthetical) - shares | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Noncontrolling Interests in Subsidiaries [Member] |  |
BGC's purchase of exchangeable limited partnership units in Newmark Holdings, shares | 16,600,000 |
Class A Common Stock [Member] |  |
Issuance of shares in the Separation, shares |  | 115,593,787
Proceeds from IPO, net of underwriting discounts and other expenses, shares |  | 23,000,000
Equity-based compensation and related issuance, shares | 327,746 | 600,000
Class B Common Stock [Member] |  |
Issuance of shares in the Separation, shares |  | 15,840,049

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