Financial Instruments Measured at Fair Value by Level Within Fair Value Hierarchy (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	$ 26,575	$ 26,596
Money market funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	18	17
U.S. government and agency securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	26,557	26,579
Level I
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	18	17
Level I | Money market funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	18	17
Level I | U.S. government and agency securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	0	0
Level II
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	26,557	26,579
Level II | Money market funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	0	0
Level II | U.S. government and agency securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	26,557	26,579
Level III
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	0	0
Level III | Money market funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	0	0
Level III | U.S. government and agency securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	$ 0	$ 0