Depreciation and Amortization - Depreciation and Amortization Expense Excluded from Operating Expenses in its Statements of Income and Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Depreciation and Amortization Expense [Line Items]
Depreciation and amortization	$ 55,089	$ 51,796	$ 167,251	$ 155,003
Direct Advertising Expenses [Member]
Depreciation and Amortization Expense [Line Items]
Depreciation and amortization	51,360	47,168	155,832	144,171
General and Administrative Expenses [Member]
Depreciation and Amortization Expense [Line Items]
Depreciation and amortization	927	955	2,935	2,857
Corporate Expenses [Member]
Depreciation and Amortization Expense [Line Items]
Depreciation and amortization	$ 2,802	$ 3,673	$ 8,484	$ 7,975