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