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Fair Value Measurements - Change in Level 3 Fair Values of Contingent Purchase Price Liabilities (Detail) - Level 3 [Member] - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning balance $ (37,574)  
Ending balance (44,624)  
Contingent Purchase Price Liabilities [Member]    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning balance (37,574) $ (33,709)
Additions from business acquisitions (8,320) (1,661)
Settlement of contingent purchase price liabilities 2,879 3,564
Change in fair value of contingencies (1,370) (485)
Change in net present value of contingencies (239) (131)
Ending balance $ (44,624) $ (32,422)