Other Assets and Liabilities - Summary of Components of Other Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Other Liabilities [Abstract]
Accrued dividends payable	$ 74,195	$ 66,888
Accrued interest payable	23,701	14,162
Accrued management and incentive fees payable	18,368	14,284
Derivative liabilities	3,676	4,911
Accounts payable and other liabilities	3,426	2,125
Secured debt repayments pending servicer remittance		38,456
Total	$ 123,366	$ 140,826