Fair Value Hierarchy for Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) (Fair Value, Measurements, Recurring, USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	$ 145,614	$ 111,671
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	18,862	17,171
US Government Agencies Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	102,174	35,920
Corporate Debt Securities, Secured by U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	24,578	58,580
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	18,862	17,171
Fair Value, Inputs, Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	18,862	17,171
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	126,752	94,500
Fair Value, Inputs, Level 2 | US Government Agencies Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	102,174	35,920
Fair Value, Inputs, Level 2 | Corporate Debt Securities, Secured by U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	$ 24,578	$ 58,580