Geographic Concentrations Based on Unpaid Principal Balances (Detail) - Residential Mortgage Loans - Geographic Concentration Risk	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate [Line Items]
Percent of Balance	100.00%	100.00%
CALIFORNIA
Mortgage Loans on Real Estate [Line Items]
Percent of Balance	57.90%	46.30%
NEW YORK
Mortgage Loans on Real Estate [Line Items]
Percent of Balance	7.20%
TEXAS
Mortgage Loans on Real Estate [Line Items]
Percent of Balance	5.00%	9.60%
Other
Mortgage Loans on Real Estate [Line Items]
Percent of Balance	29.90%	28.10%
ILLINOIS
Mortgage Loans on Real Estate [Line Items]
Percent of Balance		5.70%
FLORIDA
Mortgage Loans on Real Estate [Line Items]
Percent of Balance		5.20%
WASHINGTON
Mortgage Loans on Real Estate [Line Items]
Percent of Balance		5.10%