Schedule of Investment in Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investments And Fair Value Measurements [Line Items]
Fair Value	$ 50,433	$ 65,511
Gross Unrealized Losses	(18)	(39)
Fair Value	2,999	1,999
Gross Unrealized Losses	(2)
Total Fair Value	53,432	67,510
Total Gross Unrealized Losses	(20)	(39)
US Government Agencies Debt Securities
Investments And Fair Value Measurements [Line Items]
Fair Value	11,334	21,768
Gross Unrealized Losses	(1)	(7)
Fair Value
Gross Unrealized Losses
Total Fair Value	11,334	21,768
Total Gross Unrealized Losses	(1)	(7)
Corporate Debt Securities
Investments And Fair Value Measurements [Line Items]
Fair Value	39,099	43,743
Gross Unrealized Losses	(17)	(32)
Fair Value	2,999	1,999
Gross Unrealized Losses	(2)
Total Fair Value	42,098	45,742
Total Gross Unrealized Losses	$ (19)	$ (32)