Credit Enhancement Receivable, Contingent Liability & Credit Enhancement Deposit - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit [Line Items]
Credit enhancement agreement date			Mar. 09, 2012
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		$ 25,000,000
Credit Risk [Member]
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit [Line Items]
Credit risk loans	19,200,000,000		19,200,000,000	$ 18,800,000,000
Maximum pre-credit enhancement loss exposure			5,400,000,000	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	343,200,000		343,200,000	4,200,000,000
Maximum pre-credit enhancement loss exposure			103,700,000	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		$ 0	10,000
Security deposit return date			Mar. 09, 2021
Percentage of contingent payment	50.00%		50.00%
Contingent payment due date			Mar. 09, 2021
Contingent payments description			the Company 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) the Company’s unreimbursed loss sharing payments from March 9, 2012 through March 9, 2021 on the Company’s servicing portfolio as of March 9, 2012.
Contingent liability	$ 10,800,000		$ 10,800,000	$ 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		$ 25,000,000