Short -Term Investments (Details 2)	Dec. 31, 2012	Dec. 31, 2011
Commercial Paper [Member]
Average coupon rate percentage and the average days to maturity of debt
Average Coupon Rate	0.305%	0.191%
Average Days to Maturity	138 days	116 days
Corporate bonds and notes [Member]
Average coupon rate percentage and the average days to maturity of debt
Average Coupon Rate	2.965%	4.921%
Average Days to Maturity	112 days	228 days