Schedule of Cash, Cash Equivalents and Available-for-Sale Securities by Significant Investment Category (Parenthetical) (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Minimum
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Long-term marketable securities maturity period	1 year	1 year
Maximum
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Short-term marketable securities maturity period	1 year	1 year
Long-term marketable securities maturity period	5 years	5 years