Other Expenses - Schedule of General and Administrative Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Professional services	$ 1,223	$ 1,017
Operating and other costs	509	445
Subtotal	1,732	1,462
Non-cash compensation expenses
Restricted class A common stock earned	6,851	5,813
Subtotal	6,976	5,907
General and administrative expenses	8,708	7,428
Other expenses		59
General and Administrative Expense [Member]
Non-cash compensation expenses
Director stock-based compensation	125	94
Manager [Member]
Non-cash compensation expenses
General and administrative expenses	$ 8,708	$ 7,369