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Available-for-Sale Securities - Additional Information (Detail) (USD $)
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Schedule of Available-for-sale Securities [Line Items]    
Gross realized gains from the sale or maturity of available-for-sale investments $ 0 $ 0
Gross realized losses excluding other-than-temporary impairments from the sale or maturity of available-for-sale investments 0 0
Other-than-temporary impairments from the sale or maturity of available-for-sale investments $ 0 $ 0