Regulatory Requirements - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Minimum period required for financial resources to cover operating costs	1 year
Minimum period required for cash or highly liquid securities to cover operating costs	6 months
Net assets held by regulated subsidiaries	$ 535,300,000
Amount of capital in excess of aggregate regulatory requirements	$ 308,200,000
Percentage of Freddie Mac's liquidity requirement of outstanding principal of TAH loans serviced	8.00%
Other Assets [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Outstanding borrower advances	$ 440,000	$ 120,400
Seller/Servicer Agreements [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Amount of capital in excess of aggregate regulatory requirements	$ 475,100,000