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Investments (Tables)
 12 Months Ended
Dec. 31, 2012
Available-for-sale Securities [Abstract]  
Summary of Short-Term Investments
The following is a summary of our investments (in thousands):
 
 
December 31, 2012
 
Amortized
Cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Estimated
Fair Value
Available-for-sale securities:
 
 
 
 
 
 
 
Commercial paper
$
72,850

 
$
—

 
$
(15
)
 
$
72,835

Corporate notes and bonds
158,038

 
8

 
(98
)
 
157,948

U.S. government agency securities
1,001

 
—

 
—

 
1,001

Total available-for-sale securities
$
231,889

 
$
8

 
$
(113
)
 
$
231,784
Fair Values and Gross Unrealized Losses of Available-for-Sale Securities Aggregated by Investment Category
The following table shows the fair values and the gross unrealized losses of these available-for-sale securities aggregated by investment category (in thousands):
 
 
December 31, 2012
 
Fair Value
 
Gross Unrealized
Losses
Commercial paper
$
36,753

 
$
(15
)
Corporate notes and bonds
137,558

 
(98
)
Total
$
174,311

 
$
(113
)