Distribution Agreement - Additional Information (Detail) - Itochu Corporation Agreement $ in Millions	1 Months Ended		12 Months Ended			24 Months Ended
	Aug. 31, 2016 USD ($)	Dec. 31, 2014 USD ($) Installment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2015 USD ($)
Distribution Agreement [Line Items]
Distribution agreement term		10 years
Distribution fees			$ 0.5	$ 0.5	$ 0.5
Number of installments | Installment		3
Remaining term of distribution agreement	8 years 6 months
Distribution Rights
Distribution Agreement [Line Items]
Distribution fees		$ 4.0
Distribution fee reclassified to deferred revenue	$ 4.0
First Installment | Distribution Rights
Distribution Agreement [Line Items]
Distribution fees		1.0
Second Installment | Distribution Rights
Distribution Agreement [Line Items]
Distribution fees		1.0
Third Installment
Distribution Agreement [Line Items]
Distribution fees payment	$ 2.0
Third Installment | Distribution Rights
Distribution Agreement [Line Items]
Distribution fees		$ 2.0
First and Second Installment
Distribution Agreement [Line Items]
Distribution fees payment						$ 2.0