Changes in Level Three Instruments Consisting of Auction Rate Securities Classified As Available-for-Sale Securities and Measured on Recurring Basis (Detail) (Fair Value, Measurements, Recurring, Auction rate securities ("ARS"), Fair Value, Inputs, Level 3, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012
Fair Value, Measurements, Recurring | Auction rate securities ("ARS") | Fair Value, Inputs, Level 3
Fair Value, Instruments Classified in Shareholders' Equity Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Opening balance	$ 4,490
Total gains for the period included in other comprehensive income
Closing balance	$ 4,490