Depreciation and Amortization - Depreciation and Amortization Expense for Property and Equipment and Intangible Assets (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Property, Plant and Equipment [Line Items]
Total depreciation expense	$ 5,852	$ 5,888	$ 17,528	$ 17,167
Operating Expenses [Member]
Property, Plant and Equipment [Line Items]
Total depreciation expense	5,794	5,796	17,301	16,885
Corporate General and Administrative Expenses [Member]
Property, Plant and Equipment [Line Items]
Total depreciation expense	$ 58	$ 92	$ 227	$ 282