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Available-for-Sale Securities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains from the sale or maturity of available-for-sale investments	$ 0	$ 0	$ 0
Gross realized losses excluding other-than-temporary impairments from the sale or maturity of available-for-sale investments	0	0	0
Other-than-temporary impairments from the sale or maturity of available-for-sale investments	$ 0	$ 0	$ 0

The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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