Distribution Agreement - Additional Information (Detail) $ in Thousands		1 Months Ended		3 Months Ended		6 Months Ended		24 Months Ended
	Aug. 31, 2016 USD ($)	Aug. 31, 2016 USD ($)	Dec. 31, 2014 USD ($) Installment	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)
Marketing Agreement [Line Items]
Distribution fee reclassified to deferred revenue				$ 19,933		$ 19,933			$ 23,389
Distribution Rights
Marketing Agreement [Line Items]
Distribution fee reclassified to deferred revenue				$ 3,108		$ 3,108			$ 3,346
Itochu Corporation Agreement
Marketing Agreement [Line Items]
Distribution agreement term			10 years
Number of installments | Installment			3
Remaining term of distribution agreement	8 years 6 months			8 years 6 months		8 years 6 months
Distribution rights revenue				$ 119	$ 119	$ 238	$ 238
Itochu Corporation Agreement | Distribution Rights
Marketing Agreement [Line Items]
Distribution fees			$ 4,000
Distribution fee reclassified to deferred revenue	$ 4,000	$ 4,000
Itochu Corporation Agreement | First Installment | Distribution Rights
Marketing Agreement [Line Items]
Distribution fees			1,000
Itochu Corporation Agreement | Second Installment | Distribution Rights
Marketing Agreement [Line Items]
Distribution fees			1,000
Itochu Corporation Agreement | Third Installment
Marketing Agreement [Line Items]
Distribution fees payment		$ 2,000
Itochu Corporation Agreement | Third Installment | Distribution Rights
Marketing Agreement [Line Items]
Distribution fees			$ 2,000
Itochu Corporation Agreement | First and Second Installment
Marketing Agreement [Line Items]
Distribution fees payment								$ 2,000