Short-Term and Long-Term Investments - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Unrealized Losses On Short Term And Long Term Investments [Line Items]
Amortized cost of securities	$ 3,340,000	$ 3,700,000
Minimum period for which failed auction securities been in unrealized loss position	12 months
Estimated Fair Value	$ 2,970,000	3,272,000
Gross unrealized losses	371,000	428,000
Aggregate credit loss	72,000	84,000	$ 356,000	$ 395,000
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	$ 2,629,000	2,575,000
Gross unrealized losses	371,000	$ 425,000
Aggregate credit loss	72,000
Aggregate temporary impairment loss	$ 299,000