Variable Interest Entities - Summary of Geographic Concentrations of Residential Mortgage Loans Held-for-Investment (Detail) - Variable Interest Entities Primary Beneficiary - Geographic Concentration Risk - Residential Loans Held For Investment	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Concentration risk percentage	100.00%	100.00%
California
Concentration Risk [Line Items]
Concentration risk percentage	43.90%	42.90%
Florida
Concentration Risk [Line Items]
Concentration risk percentage	6.30%	8.40%
Other states
Concentration Risk [Line Items]
Concentration risk percentage	49.80%	48.70%