Cash Equivalents and Investments (Footnote) (Detail) - Certificates of deposit and money market deposit accounts	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Short-term Investments | Minimum
Cash and Cash Equivalents [Line Items]
Securities maturity period	1 month
Short-term Investments | Maximum
Cash and Cash Equivalents [Line Items]
Securities maturity period		12 months
Other assets | Minimum
Cash and Cash Equivalents [Line Items]
Securities maturity period	13 months	13 months
Other assets | Maximum
Cash and Cash Equivalents [Line Items]
Securities maturity period	24 months	23 months
Cash and Cash Equivalents | Maximum
Cash and Cash Equivalents [Line Items]
Securities maturity period	90 days