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 6 Months Ended
Jun. 30, 2012
Jul. 02, 2011
Jun. 30, 2012
Jul. 02, 2011
Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]          
Maturity of investments, maximum (in years)     2 years    
Amortized cost $ 143.0   $ 143.0   $ 169.7
Gross unrealized gains 0   0   0
Gross unrealized losses (0.1)   (0.1)   (0.1)
Fair value (net carrying amount) 142.9   142.9   169.6
Available-for-sale debt securities [Abstract]          
Due in one year or less, amortized cost 96.5   96.5   76.7
Due after one year through two years, amortized cost 46.5   46.5   93.0
Total available-for-sale debt securities, amortized cost 143.0   143.0   169.7
Due in one year or less, fair value (net carrying amount) 96.4   96.4   76.7
Due after one year through two years, fair value (net carrying amount) 46.5   46.5   92.9
Total available-for-sale debt securities, fair value (net carrying amount) 142.9   142.9   169.6
Sales of available-for-sale securities     74.2 55.3  
Redemptions of available-for-sale securities     74.0 24.0  
Net adjustment to unrealized investment losses on available-for-sale securities 0.1 0 0 (0.1)  
Agency Bonds [Member]
          
Schedule of Available-for-sale Securities [Line Items]          
Amortized cost 46.5   46.5   97.7
Gross unrealized gains 0   0   0
Gross unrealized losses 0   0   (0.1)
Fair value (net carrying amount) 46.5   46.5   97.6
Corporate Bonds [Member]
          
Schedule of Available-for-sale Securities [Line Items]          
Amortized cost 74.7   74.7   51.7
Gross unrealized gains 0   0   0
Gross unrealized losses (0.1)   (0.1)   0
Fair value (net carrying amount) 74.6   74.6   51.7
Commercial Paper [Member]
          
Schedule of Available-for-sale Securities [Line Items]          
Amortized cost 21.0   21.0   19.5
Gross unrealized gains 0   0   0
Gross unrealized losses 0   0   0
Fair value (net carrying amount) 21.0   21.0   19.5
U.S. Treasury Bills [Member]
          
Schedule of Available-for-sale Securities [Line Items]          
Amortized cost 0.8   0.8   0.8
Gross unrealized gains 0   0   0
Gross unrealized losses 0   0   0
Fair value (net carrying amount) $ 0.8   $ 0.8   $ 0.8