Geographic Concentrations - Additional Information (Detail) - Geographic Concentration Risk - Country	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Non-US | Sales Revenue, Segment
Concentration Risk [Line Items]
Number of foreign countries representing more than ten percent	0	0	0
Non-US | Long-lived Assets
Concentration Risk [Line Items]
Number of foreign countries representing more than ten percent	0	0
Minimum | U.S. | Sales Revenue, Segment
Concentration Risk [Line Items]
Concentration risk	90.00%	90.00%	90.00%
Minimum | U.S. | Long-lived Assets
Concentration Risk [Line Items]
Concentration risk	85.00%	87.00%