Fair Value Measurements - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value, assets, level 1 to level 2 transfers, amount	$ 0	$ 0
Fair value, assets, level 2 to level 1 transfers, amount	0	0
Fair value, assets, transfers into level 3, amount	0	0
Fair value, assets, transfers out of level 3, amount	0	0
Contingent Consideration [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent consideration liability	$ 12,079,000	$ 11,948,000	$ 9,708,000
Serendex A/S [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Business combination completion date	Jul. 15, 2016
European Medicines Agency Approval [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent milestone payments	$ 5,000,000
FDA Approval [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent milestone payments	15,000,000
Japanese Pharmaceuticals and Medical Devices Agency Approval [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent milestone payments	$ 1,500,000