Gross Unrealized Losses and Fair Values of the Company's Short-Term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 36,140	$ 16,892
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(425)	(316)
Available-for-sale Securities, Continuous Unrealized Loss Position, Greater than 12 Months, Fair Value	2,165	1,806
Available-for-sale Securities, Continuous Unrealized Loss Position, Greater than 12 Months, Unrealized Losses	(49)	(92)
Asset-backed and mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	29,041	10,645
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(424)	(286)
Available-for-sale Securities, Continuous Unrealized Loss Position, Greater than 12 Months, Fair Value	2,165	1,301
Available-for-sale Securities, Continuous Unrealized Loss Position, Greater than 12 Months, Unrealized Losses	(49)	(92)
U.S. government and agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	7,099	3,872
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(1)	(3)
Available-for-sale Securities, Continuous Unrealized Loss Position, Greater than 12 Months, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Greater than 12 Months, Unrealized Losses	0	0
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value		2,375
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses		(27)
Available-for-sale Securities, Continuous Unrealized Loss Position, Greater than 12 Months, Fair Value		505
Available-for-sale Securities, Continuous Unrealized Loss Position, Greater than 12 Months, Unrealized Losses		$ 0