Depreciation and Amortization - Depreciation and Amortization Expense for Property and Equipment and Intangible Assets (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Property, Plant and Equipment [Line Items]
Total depreciation expense	$ 5,901	$ 5,638	$ 11,676	$ 11,279
Operating Expenses [Member]
Property, Plant and Equipment [Line Items]
Total depreciation expense	5,825	5,546	11,507	11,089
Corporate General and Administrative Expenses [Member]
Property, Plant and Equipment [Line Items]
Total depreciation expense	$ 76	$ 92	$ 169	$ 190