Investments (Details 2)	Mar. 31, 2013	Dec. 31, 2012
Commercial Paper [Member]
Average coupon rate percentage and the average days to maturity of debt
Average Coupon Rate	0.163%	0.305%
Average Days to Maturity	116 days	138 days
Corporate bonds and notes [Member]
Average coupon rate percentage and the average days to maturity of debt
Average Coupon Rate	3.041%	2.965%
Average Days to Maturity	108 days	112 days