Other Long-Term Liabilities - Summary of Other Long-Term Liabilities (Parenthetical) (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Series B Convertible Preferred Stock [Member]
Other Long Term Liabilities [Line Items]
Convertible preferred stock interest rate	7.00%	7.00%	7.00%	7.00%	7.00%