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Investments (Tables)
 12 Months Ended
Dec. 31, 2016
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, 2016:

One year or less
 
55 %
After one year but within two years
 
32 %
After two years but within three and a half years
 
13 %
Total
 
100 %
Summary of Investments
The following is a summary of our investments (in thousands):

   
Gross Unrealized
   
December 31, 2016
 
Cost (1)
  
Gains
  
Losses
  
Estimated Fair Value
 
Available-for-sale securities:
            
Corporate debt securities
 
$
195,087
  
$
25
  
$
(161
)
 
$
194,951
 
Debt securities issued by U.S. government agencies
  
26,548
   
—
   
(10
)
  
26,538
 
Debt securities issued by the U.S. Treasury
  
29,298
   
2
   
(14
)
  
29,286
 
Debt securities issued by states of the U.S. and political subdivisions of the states (2)
  
72,775
   
2
   
(134
)
  
72,643
 
Total securities with a maturity of one year or less
  
323,708
   
29
   
(319
)
  
323,418
 
Corporate debt securities
  
202,408
   
36
   
(1,174
)
  
201,270
 
Debt securities issued by U.S. government agencies
  
28,807
   
1
   
(167
)
  
28,641
 
Debt securities issued by states of the U.S. and political subdivisions of the states
  
36,816
   
1
   
(349
)
  
36,468
 
Total securities with a maturity of more than one year
  
268,031
   
38
   
(1,690
)
  
266,379
 
Total available-for-sale securities
 
$
591,739
  
$
67
  
$
(2,009
)
 
$
589,797
 
Equity securities:
                
Regulus Therapeutics Inc.
 
$
2,133
  
$
281
  
$
—
  
$
2,414
 
Total equity securities
 
$
2,133
  
$
281
  
$
—
  
$
2,414
 
Total available-for-sale and equity securities
 
$
593,872
  
$
348
  
$
(2,009
)
 
$
592,211
 




   
Gross Unrealized
   
December 31, 2015
 
Cost (1)
  
Gains
  
Losses
  
Estimated
Fair Value
 
Available-for-sale securities:
            
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 U.S. 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 U.S. 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
 

(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, 2016 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
  
255
  
$
280,393
  
$
(1,197
)
 
$
32,753
  
$
(138
)
 
$
313,146
  
$
(1,335
)
Debt securities issued by U.S. government agencies
  
29
   
43,851
   
(177
)
  
—
   
—
   
43,851
   
(177
)
Debt securities issued by the U.S. Treasury
  
2
   
18,782
   
(14
)
  
—
   
—
   
18,782
   
(14
)
Debt securities issued by states of the U.S. and political subdivisions of the states
  
135
   
80,896
   
(398
)
  
6,934
   
(85
)
  
87,830
   
(483
)
Total temporarily impaired securities
  
421
  
$
423,922
  
$
(1,786
)
 
$
39,687
  
$
(223
)
 
$
463,609
  
$
(2,009
)