Securitized Debt - Maturities (Detail) - Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Loans held for investment
Debt Instrument [Line Items]
Within One Year	$ 1,785,710	$ 1,819,018
One to Three Years	2,818,626	2,872,572
Three to Five Years	2,110,377	2,140,434
Greater Than Five Years	2,493,804	2,439,872
Total	9,208,517	9,271,896
Securitized Loans | Non-agency MBS
Debt Instrument [Line Items]
Within One Year	30,923	42,217
One to Three Years	37,753	45,916
Three to Five Years	11,461	16,524
Greater Than Five Years	5,600	4,867
Total	$ 85,737	$ 109,524