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Acquisitions (Detail) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Sep. 30, 2017
Other Payments to Acquire Businesses $ 901,000        
Payments to Acquire Businesses, Gross 37,112,000        
Payments to Acquire Businesses, Net of Cash Acquired 17,396,000 $ 0 $ 16,795,000 $ 0  
Business Acquisition, Contingent Consideration, at Fair Value $ 20,617,000 $ 0 20,617,000   $ 10,000,000
Business Combination, Assets and Liabilities Arising from Contingencies, Amount Recognized, Net     $ 10,617,000