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Fair Value Measurements - Additional Information (Detail) - USD ($)
8 Months Ended 12 Months Ended
Feb. 14, 2019
Apr. 30, 2018
Dec. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Feb. 06, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Amortized cost basis     $ 9,000,000.0 $ 9,000,000.0      
Other-than-temporary impairment on debt securities         $ 5,585,000 $ 2,727,000  
Changes in fair value of contingent consideration       3,069,000      
Spinal Kinetics [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Milestone achievement period   5 years          
Contingent consideration   $ 25,500,000 28,600,000 28,600,000      
Spinal Kinetics [Member] | Operating Expense [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Changes in fair value of contingent consideration     3,100,000 1,400,000      
Other Current Liabilities [Member] | Spinal Kinetics [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Contingent consideration     13,600,000 13,600,000      
Other Long-term Liabilities [Member] | Spinal Kinetics [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Contingent consideration     15,000,000.0 15,000,000.0      
Maximum [Member] | Spinal Kinetics [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Future milestone payments   60,000,000.0          
US Food And Drug Administration [Member] | Spinal Kinetics [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Future milestone payments   15,000,000.0          
US Food And Drug Administration [Member] | Spinal Kinetics [Member] | Subsequent Event [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Future milestone payments             $ 15,000,000.0
Payments obligation for contigent consideration $ 15,000,000.0            
Revenue Milestone [Member] | Spinal Kinetics [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Future milestone payments   $ 45,000,000.0          
Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Fair value of debt securities     $ 17,820,000 17,820,000 $ 16,050,000 $ 12,220,000  
Net increase in estimated fair value of the debt securities       1,800,000      
Common Stock [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Warrants’ value Reduced       $ 0