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Investments (Tables)
 12 Months Ended
Dec. 31, 2015
Investments [Abstract]  
Contract Maturity of Available-for-Sale Securities
The following table summarizes the contract maturity of the available-for-sale securities we held as of December 31, 2015:

One year or less
 
49 %
After one year but within two years
 
27 %
After two years but within three and a half years
 
24 %
Total
 
100 %
Summary of Investments
The following is a summary of our investments (in thousands):

   
Gross Unrealized
   
December 31, 2015
 
Cost
  
Gains
  
Losses
  
Estimated
Fair Value
 
Available-for-sale securities (1):
        
Corporate debt securities
 
$
181,670
  
$
5
  
$
(250
)
 
$
181,425
 
Debt securities issued by U.S. government agencies
  
50,559
   
1
   
(19
)
  
50,541
 
Debt securities issued by the U.S. Treasury
  
2,604
   
—
   
(3
)
  
2,601
 
Debt securities issued by states of the United States and political subdivisions of the states (2)
  
79,414
   
18
   
(88
)
  
79,344
 
Total securities with a maturity of one year or less
  
314,247
   
24
   
(360
)
  
313,911
 
Corporate debt securities
  
258,703
   
3
   
(1,705
)
  
257,001
 
Debt securities issued by U.S. government agencies
  
38,956
   
—
   
(244
)
  
38,712
 
Debt securities issued by states of the United States and political subdivisions of the states
  
48,552
   
3
   
(243
)
  
48,312
 
Total securities with a maturity of more than one year
  
346,211
   
6
   
(2,192
)
  
344,025
 
Total available-for-sale securities
 
$
660,458
  
$
30
  
$
(2,552
)
 
$
657,936
 
Equity securities:
                
Regulus Therapeutics Inc.
 
$
7,162
  
$
17,630
  
$
—
  
$
24,792
 
Total equity securities
 
$
7,162
  
$
17,630
  
$
—
  
$
24,792
 
Total available-for-sale and equity securities
 
$
667,620
  
$
17,660
  
$
(2,552
)
 
$
682,728
 

   
Gross Unrealized
   
December 31, 2014
 
Cost
  
Gains
  
Losses
  
Estimated
Fair Value
 
Available-for-sale securities (1):
        
Corporate debt securities (2)
 
$
219,856
  
$
89
  
$
(89
)
 
$
219,856
 
Debt securities issued by U.S. government agencies
  
47,496
   
7
   
(27
)
  
47,476
 
Debt securities issued by the U.S. Treasury (2)
  
19,008
   
9
   
—
   
19,017
 
Debt securities issued by states of the United States and political subdivisions of the states (2)
  
45,196
   
19
   
(53
)
  
45,162
 
Total securities with a maturity of one year or less
  
331,556
   
124
   
(169
)
  
331,511
 
Corporate debt securities
  
152,730
   
16
   
(600
)
  
152,146
 
Debt securities issued by U.S. government agencies
  
62,530
   
—
   
(151
)
  
62,379
 
Debt securities issued by states of the United States and political subdivisions of the states
  
60,073
   
32
   
(234
)
  
59,871
 
Total securities with a maturity of more than one year
  
275,333
   
48
   
(985
)
  
274,396
 
Total available-for-sale securities
 
$
606,889
  
$
172
  
$
(1,154
)
 
$
605,907
 
Equity securities:
                
Regulus Therapeutics Inc.
 
$
12,477
  
$
69,404
  
$
—
  
$
81,881
 
Total equity securities
 
$
12,477
  
$
69,404
  
$
—
  
$
81,881
 
Total available-for-sale and equity securities
 
$
619,366
  
$
69,576
  
$
(1,154
)
 
$
687,788
 

(1)Our available-for-sale securities are held at amortized cost.

(2)Includes investments classified as cash equivalents on our consolidated balance sheet.
Temporarily Impaired Investments
Investments we consider to be temporarily impaired at December 31, 2015 are as follows (in thousands):

    
Less than 12 months of
Temporary Impairment
  
More than 12 months of
Temporary Impairment
  
Total Temporary
Impairment
 
  
Number of
Investments
  
Estimated
Fair Value
  
Unrealized
Losses
  
Estimated
Fair Value
  
Unrealized
Losses
  
Estimated
Fair Value
  
Unrealized
Losses
 
Corporate debt securities
  
397
  
$
406,098
  
$
(1,908
)
 
$
16,066
  
$
(47
)
 
$
422,164
  
$
(1,955
)
Debt securities issued by U.S. government agencies
  
19
   
71,842
   
(262
)
  
1,001
   
(1
)
  
72,843
   
(263
)
Debt securities issued by the U.S. Treasury
  
2
   
2,601
   
(3
)
  
—
   
—
   
2,601
   
(3
)
Debt securities issued by states of the United States and political subdivisions of the states
  
230
   
59,882
   
(173
)
  
29,634
   
(158
)
  
89,516
   
(331
)
Total temporarily impaired securities
  
648
  
$
540,423
  
$
(2,346
)
 
$
46,701
  
$
(206
)
 
$
587,124
  
$
(2,552
)