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Fair Value Measurements - Change in Level 3 Fair Values of Contingent Purchase Price Liabilities (Detail) - Contingent Purchase Price Liabilities [Member] - Level 3 [Member] - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning balance $ (39,708) $ (37,574)
Additions from business acquisitions (10,150) (12,361)
Settlement of contingent purchase price liabilities 18,311 13,329
Change in fair value of contingencies 222 (2,574)
Change in net present value of contingencies (544) (716)
Ending balance $ (31,869) $ (39,896)