Short-Term and Long-Term Investments - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Unrealized Losses On Short Term And Long Term Investments [Line Items]
Amortized cost of securities	$ 3,700,000	$ 3,700,000	$ 6,820,000
Minimum period for which failed auction securities been in unrealized loss position		12 months
Estimated fair value of securities	3,272,000	3,272,000	5,651,000
Redemption, net	3,000,000
Gross unrealized losses	428,000	428,000	1,175,000
Aggregate credit loss	84,000	84,000	395,000	317,000	308,000
Failed Auction Securities [Member]
Unrealized Losses On Short Term And Long Term Investments [Line Items]
Amortized cost of securities	3,000,000	3,000,000	6,000,000
Period for which failed auction securities been in unrealized loss position		Exceeds 12 months
Estimated fair value of securities	2,575,000	2,575,000	4,825,000
Gross unrealized losses	425,000	425,000	1,175,000
Aggregate credit loss	84,000	84,000
Aggregate temporary impairment loss		$ 341,000