Other Expenses - Schedule of General and Administrative Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Professional services	$ 1,249	$ 1,139	$ 2,439	$ 2,361
Operating and other costs	894	830	1,249	1,339
Subtotal	2,143	1,969	3,688	3,700
Non-cash compensation expenses
Restricted class A common stock earned	7,629	6,653	15,272	13,504
Director stock-based compensation	125	125	250	250
Subtotal	7,754	6,778	15,522	13,754
Total general and administrative expenses	$ 9,897	$ 8,747	$ 19,210	$ 17,454