Leases - Schedule of Operating Lease Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Lessee Lease Description [Line Items]
Total lease costs	$ 1,518	$ 3,248
Operating and Maintenance [Member]
Lessee Lease Description [Line Items]
Total lease costs	1,017	2,035
General and Administrative [Member]
Lessee Lease Description [Line Items]
Total lease costs	$ 501	$ 1,213