Distribution Agreement - Additional Information (Detail) - Itochu Corporation Agreement $ in Millions		1 Months Ended		12 Months Ended			24 Months Ended
	Aug. 31, 2016 USD ($)	Aug. 31, 2016 USD ($)	Dec. 31, 2014 USD ($) Installment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Marketing Agreement [Line Items]
Distribution agreement term			10 years
Distribution fees				$ 0.5	$ 0.5	$ 0.2
Number of installments | Installment			3
Remaining term of distribution agreement	8 years 6 months	8 years 6 months		8 years 6 months
Distribution Rights
Marketing Agreement [Line Items]
Distribution fees			$ 4.0
Distribution fee reclassified to deferred revenue	$ 4.0	$ 4.0
First Installment | Distribution Rights
Marketing Agreement [Line Items]
Distribution fees			1.0
Second Installment | Distribution Rights
Marketing Agreement [Line Items]
Distribution fees			1.0
Third Installment
Marketing Agreement [Line Items]
Distribution fees payment		$ 2.0
Third Installment | Distribution Rights
Marketing Agreement [Line Items]
Distribution fees			2.0
First and Second Installment
Marketing Agreement [Line Items]
Distribution fees payment			$ 2.0				$ 2.0