Credit Enhancement Receivable, Contingent Liability and Credit Enhancement Deposit - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit [Line Items]
Credit enhancement agreement date	Mar. 09, 2012
Maximum pre-credit enhancement loss exposure	$ 5,400,000,000
Restricted Cash And Offsetting Liability [Member]
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit [Line Items]
Deposit in Fannie Mae restricted liquidity account	25,000,000
Credit Risk [Member]
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit [Line Items]
Credit risk loans	18,900,000,000		$ 18,800,000,000
Maximum pre-credit enhancement loss exposure			5,300,000,000
DB Cayman [Member]
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit [Line Items]
Reimbursements under serving agreement	0	$ 0
Credit risk loans	400,000,000		4,200,000,000
Maximum pre-credit enhancement loss exposure	100,000,000		1,200,000,000
Maximum loss exposure without any form of credit protection	5,300,000,000		4,100,000,000
Credit enhancement receivable	$ 0		10,000
Security deposit return date	Mar. 09, 2021
Percentage of contingent payment	50.00%
Contingent payment due date	Mar. 09, 2021
Contingent payments description	Newmark is required to pay DB Cayman, on March 9, 2021, an amount equal to 50% of the positive difference, if any, between (a) $25 million, and (b) Newmark’s unreimbursed loss-sharing payments from March 9, 2012 through March 9, 2021 on Newmark’s servicing portfolio as of March 9, 2012.
Contingent liability	$ 10,700,000		$ 10,700,000
DB Entities [Member]
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit [Line Items]
Deposit in Fannie Mae restricted liquidity account	$ 25,000,000