Investments (Unaudited) (Parenthetical) (Details)	Sep. 30, 2011	Dec. 31, 2010
Investments [Abstract]
Average certificates of deposit available-for-sale maturity, months	2	5
Average corporate bonds available-for-sale maturity, months	8	4
Average federal government bonds available-for-sale maturity, months	6
Average commercial paper available-for-sale maturity, months		4
Average federal government bonds held-to-maturity, months	2	9
Average corporate bonds held-to-maturity, months	2	10

The average maturity date in months of available-for-sale certificates of deposit.

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The average maturity date in months of available-for-sale commercial paper.

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The average maturity date in months of available-for-sale corporate bonds.

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The average maturity date in months of available-for-sale federal government bonds.

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The average maturity date in months of held-to-maturity corporate bonds.

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The average maturity date in months of held-to-maturity federal government bonds.

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