Other Expenses - Schedule of General and Administrative Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Professional services	$ 1,096	$ 1,223
Operating and other costs	449	509
Subtotal	1,545	1,732
Non-cash compensation expenses
Restricted class A common stock earned	7,643	6,851
Director stock-based compensation	125	125
Subtotal	7,768	6,976
General and administrative expenses	$ 9,313	$ 8,708