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Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Corporate and municipal bonds $ 53,602 $ 51,101 $ 44,573
Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Deferred compensation plan assets 86,581 85,589  
Corporate and municipal bonds 53,602 51,101  
Deferred compensation plan liabilities (86,581) (85,589)  
Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Interest rate swaps 1,719 1,131  
Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Contingent purchase price liabilities $ (44,624) $ (37,574)