Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Money market accounts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	$ 11,912	$ 14,201
US Treasury Bill Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	59,952
Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	82	82
Futures contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	40	42
Investment in gold
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	177	156
Level 1 | Money market accounts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	11,912	14,201
Level 1 | US Treasury Bill Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	59,952
Level 1 | Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	82	82
Level 1 | Futures contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	40	42
Level 1 | Investment in gold
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets	$ 177	$ 156