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Depreciation and Amortization - Depreciation and Amortization Expense Excluded from Operating Expenses in its Consolidated Statements of Income and Comprehensive Income (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Depreciation and Amortization Expense [Line Items]      
Depreciation and amortization $ 251,296 $ 250,028 $ 225,261
Direct Expenses [Member]      
Depreciation and Amortization Expense [Line Items]      
Depreciation and amortization 236,054 235,544 212,585
General and Administrative Expenses [Member]      
Depreciation and Amortization Expense [Line Items]      
Depreciation and amortization 4,996 4,416 4,134
Corporate Expenses [Member]      
Depreciation and Amortization Expense [Line Items]      
Depreciation and amortization $ 10,246 $ 10,068 $ 8,542