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Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]        
Fair value assets transfers from level 1 to level 2 $ 0 $ 0    
Fair value assets transfers from level 2 to level 1 0 0    
Fair value liabilities transfers from level 1 to level 2 0 0    
Fair value liabilities transfers from level 2 to level 1 0 0    
Fair value assets transfers Into level 3 0 0    
Fair value assets transfers out of level 3 0 0    
Fair value liabilities transfers Into level 3 0 0    
Fair value liabilities transfers out of level 3 0 0    
Level 3 [Member]        
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]        
Present value of expected payments related to contingent consideration 30,123,000 27,340,000 $ 31,460,000 $ 48,967,000
Contingent Consideration [Member] | Level 3 [Member]        
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]        
Present value of expected payments related to contingent consideration 23,087,000 23,711,000 $ 28,998,000 $ 38,713,000
Contingent Consideration [Member] | Level 3 [Member] | Accounts Payable, Accrued and Other Liabilities [Member]        
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]        
Present value of expected payments related to contingent consideration $ 26,700,000 $ 23,700,000    
Maximum [Member]        
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]        
Derivative instruments outstanding period 60 days