Variable Interest Entities - Summary of Reconciliation of Carrying Value of Residential Mortgage Loans (Detail) - Variable Interest Entities Primary Beneficiary - Residential Mortgage Backed Securities - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Balance at beginning of period	$ 723,777	$ 969,172
Deductions during period:
Collections of principal	(27,865)	(221,615)
Amortization of premium	(657)	(3,095)
Balance at end of period	$ 695,255	$ 744,462