Cash Equivalent and Investments (Parenthetical) (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Short Term Investments [Member] | Certificates Of Deposit [Member] | Minimum [Member]
Cash and Cash Equivalents [Line Items]
Securities Maturity period	1 month	1 month
Short Term Investments [Member] | Certificates Of Deposit [Member] | Maximum [Member]
Cash and Cash Equivalents [Line Items]
Securities Maturity period	12 years	12 years
Cash And Cash Equivalents [Member] | Open Ended Mutual Funds [Member] | Maximum [Member]
Cash and Cash Equivalents [Line Items]
Securities Maturity period	90 days
Other Assets [Member] | Certificates Of Deposit [Member] | Minimum [Member]
Cash and Cash Equivalents [Line Items]
Securities Maturity period	13 years	14 years
Other Assets [Member] | Certificates Of Deposit [Member] | Maximum [Member]
Cash and Cash Equivalents [Line Items]
Securities Maturity period	24 years	36 years