Distribution Agreement - Additional Information (Detail) $ in Thousands		1 Months Ended		3 Months Ended		24 Months Ended
	Aug. 31, 2016 USD ($)	Aug. 31, 2016 USD ($)	Dec. 31, 2014 USD ($) Installment	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)
Marketing Agreement [Line Items]
Distribution fee reclassified to deferred revenue				$ 13,013			$ 23,389
Distribution rights revenue				119	$ 119
Distribution Rights
Marketing Agreement [Line Items]
Distribution fee reclassified to deferred revenue				$ 3,228			$ 3,346
Itochu Corporation Agreement
Marketing Agreement [Line Items]
Distribution agreement term			10 years
Distribution fees			$ 4,000
Number of installments | Installment			3
Remaining term of distribution agreement	8 years 6 months			8 years 6 months
Distribution rights revenue				$ 119	$ 119
Itochu Corporation Agreement | Distribution Rights
Marketing Agreement [Line Items]
Distribution fee reclassified to deferred revenue	$ 4,000	$ 4,000
Itochu Corporation Agreement | First Installment
Marketing Agreement [Line Items]
Distribution fees			$ 1,000
Itochu Corporation Agreement | Second Installment
Marketing Agreement [Line Items]
Distribution fees			1,000
Itochu Corporation Agreement | Third Installment
Marketing Agreement [Line Items]
Distribution fees			$ 2,000
Distribution fees payment		$ 2,000
Itochu Corporation Agreement | First and Second Installment
Marketing Agreement [Line Items]
Distribution fees payment						$ 2,000