Long-Term Investments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Unrealized Losses On Short Term And Long Term Investments [Line Items]
Minimum period for which failed auction securities been in unrealized loss position	12 months
Failed Auction Security [Member]
Unrealized Losses On Short Term And Long Term Investments [Line Items]
Amortized cost of securities	$ 3,000,000	$ 3,000,000
Period for which failed auction securities been in unrealized loss position	Exceeds 12 months
Estimated Fair Value	$ 2,525,000	2,508,000
Gross Unrealized Losses	475,000	492,000
Aggregate credit loss	48,000	$ 59,000	$ 72,000	$ 84,000
Aggregate temporary impairment loss	$ 427,000