Other Expenses - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Management fee - percent of outstanding equity balance			1.50%
Incentive fee computation-percent of the product per agreement			20.00%
Incentive fee computation-percent of outstanding Equity per annum			7.00%
Management fees description			Manager is entitled to an incentive fee in an amount equal to the product of (i) 20% and (ii) the excess of (a) our Core Earnings (as defined in our Management Agreement) for the previous 12-month period over (b) an amount equal to 7.00% per annum multiplied by our outstanding Equity, provided that our Core Earnings over the prior three-year period is greater than zero. Core Earnings, as defined in our Management Agreement, is generally equal to our net income (loss) prepared in accordance with GAAP, excluding (i) certain non-cash items (ii) the net income (loss) related to our legacy portfolio and (iii) incentive management fees.
Management fees	$ 18,368	$ 13,243	$ 56,248	$ 40,557
Accrued management and incentive fees payable	18,368		18,368		$ 14,284
Manager [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Management fees	12,100	9,500	34,200	28,600
Total incentive compensation payments	6,300	$ 3,700	22,000	$ 11,900
Accrued management and incentive fees payable	$ 18,400		$ 18,400		$ 14,300