Investments - Additional Information (Detail) (USD $)	3 Months Ended		0 Months Ended
	Mar. 31, 2015 Securities	Mar. 31, 2014	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Realized gains (losses) on investments	$ 0	$ 0
Available-for-sale investments in a continuous loss position for more than twelve months	$ 0	$ 0
Number of individual investment securities in unrealized loss position	16		16
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment maturity date description			Less than twenty-four months	Less than twenty-four months
Long-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment maturity date description			Less than twenty-four months	Less than twenty-four months