Quarterly report pursuant to Section 13 or 15(d)

Investments (Details)

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Investments (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 30, 2013
Mar. 31, 2012
Dec. 31, 2012
Schedule of Investments [Line Items]      
Maturity of investments, maximum 2 years    
Amortized cost $ 63.2   $ 144.4
Gross unrealized gains 0   0
Gross unrealized losses 0   0
Fair value (net carrying amount) 63.2   144.4
Available-for-sale debt securities [Abstract]      
Due in one year or less, amortized cost 34.7   92.3
Due after one year through two years, amortized cost 28.5   52.1
Total available-for-sale debt securities, amortized cost 63.2   144.4
Due in one year or less, fair value (net carrying amount) 34.7   92.3
Due after one year through two years, fair value (net carrying amount) 28.5   52.1
Total available-for-sale debt securities, fair value (net carrying amount) 63.2   144.4
Redemptions of available-for-sale securities 80.6 57.5  
Sales of available-for-sale securities   52.0  
Net adjustment to unrealized investment losses on available-for-sale securities 0 (0.1)  
Agency Bonds [Member]
     
Schedule of Investments [Line Items]      
Amortized cost 35.5   59.2
Gross unrealized gains 0   0
Gross unrealized losses 0   0
Fair value (net carrying amount) 35.5   59.2
Corporate Bonds [Member]
     
Schedule of Investments [Line Items]      
Amortized cost 26.9   66.4
Gross unrealized gains 0   0
Gross unrealized losses 0   0
Fair value (net carrying amount) 26.9   66.4
Commercial Paper [Member]
     
Schedule of Investments [Line Items]      
Amortized cost     16.0
Gross unrealized gains     0
Gross unrealized losses     0
Fair value (net carrying amount)     16.0
Certificate of Deposit [Member]
     
Schedule of Investments [Line Items]      
Amortized cost     2.0
Gross unrealized gains     0
Gross unrealized losses     0
Fair value (net carrying amount)     2.0
U.S. Treasury Bills [Member]
     
Schedule of Investments [Line Items]      
Amortized cost 0.8   0.8
Gross unrealized gains 0   0
Gross unrealized losses 0   0
Fair value (net carrying amount) $ 0.8   $ 0.8