Cash Equivalent and Investments (Parenthetical) (Detail) - Certificates Of Deposit [Member]	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Short Term Investments [Member] | Minimum [Member]
Cash and Cash Equivalents [Line Items]
Securities Maturity period	1 month	1 month
Short Term Investments [Member] | Maximum [Member]
Cash and Cash Equivalents [Line Items]
Securities Maturity period	12 months	12 months
Cash And Cash Equivalents [Member] | Maximum [Member]
Cash and Cash Equivalents [Line Items]
Securities Maturity period	90 days	90 days
Other Assets [Member] | Minimum [Member]
Cash and Cash Equivalents [Line Items]
Securities Maturity period	13 months	14 months
Other Assets [Member] | Maximum [Member]
Cash and Cash Equivalents [Line Items]
Securities Maturity period	32 months	36 months