Segment Reporting, Geographic Information, and Significant Customers - Summary of Geographic Revenue as Percentage of Total Revenues by Country (Detail) - Net Revenues - Geographic Concentration Risk	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Concentration Risk [Line Items]
Concentration risk	100.00%	100.00%	100.00%
Japan
Concentration Risk [Line Items]
Concentration risk	39.00%	11.00%	14.00%
United States of America
Concentration Risk [Line Items]
Concentration risk	22.00%	26.00%	27.00%
Korea
Concentration Risk [Line Items]
Concentration risk	18.00%	47.00%	46.00%
Countries of which none are more than 10% in a year
Concentration Risk [Line Items]
Concentration risk	21.00%	16.00%	13.00%