Fair Value Measurements - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended			5 Months Ended	9 Months Ended
	Apr. 30, 2018	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Purchase of common stock, value				$ 0		$ 500	$ 0
Investment impairment charges		$ (4,449)				(4,678)
Amortized cost basis in debt security		9,000			$ 9,000	9,000
Increase in the fair value of a contingent consideration		1,580				2,689
Spinal Kinetics [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	5 years
Contingent consideration	$ 25,491	28,200			28,200	28,200
Other Current Liabilities [Member] | Spinal Kinetics [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent consideration		13,600			13,600	13,600
Other Long-term Liabilities [Member] | Spinal Kinetics [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent consideration		14,600			14,600	14,600
Operating Expense [Member] | Spinal Kinetics [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Increase in the fair value of a contingent consideration					2,700
Maximum [Member] | Spinal Kinetics [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Future milestone payments	60,000
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
Revenue Milestone [Member] | Spinal Kinetics [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Future milestone payments	$ 45,000
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		19,250		9,000	$ 19,250	19,250	9,000	$ 16,050	$ 12,220
Increase in estimated fair value of the debt security						3,200
ASU 2016-01 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Increase in previously recorded value of equity investments resulted from new measurement				$ 0		$ 1,629	$ 0
Bone Biologics Inc [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Purchase of common stock, value			$ 500
Bone Biologics Inc [Member] | ASU 2016-01 [Member] | Equity Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Increase in previously recorded value of equity investments resulted from new measurement			$ 1,600
Impact on remeasurement of equity investments per share before taxes included within other income or expense			$ 0.09
Bone Biologics Inc [Member] | Common Stock [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Purchase of stock, shares			25,000
Bone Biologics Inc [Member] | CommonStock and Warrants [Member] | ASU 2016-01 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment impairment charges		$ 4,400