Assets Measured and Reported at Fair Value on Recurring Basis Using Significant Level 3 Inputs (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 15,630	$ 21,000
Unrealized gain (loss)	3,626	(5,370)
Transfers to Level 3			21,000
Balance at end of period	$ 19,256	$ 15,630	$ 21,000