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Investments (Tables)
12 Months Ended
Dec. 31, 2017
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, 2017:

One year or less
  
71
%
After one year but within two years
  
23
%
After two years but within three and one half years
  
6
%
Total
  
100
%

Summary of Investments
The following is a summary of our investments (in thousands):

     
Gross Unrealized
  
Estimated
 
December 31, 2017
 
Cost (1)
  
Gains
  
Losses
  
Fair Value
 
Available-for-sale securities:
            
Corporate debt securities (2)
 
$
500,599
  
$
2
  
$
(752
)
 
$
499,849
 
Debt securities issued by U.S. government agencies
  
83,926
   
   
(212
)
  
83,714
 
Debt securities issued by the U.S. Treasury
  
29,428
   
   
(17
)
  
29,411
 
Debt securities issued by states of the U.S. and political subdivisions of the states (2)
  
29,240
   
4
   
(122
)
  
29,122
 
Total securities with a maturity of one year or less
  
643,193
   
6
   
(1,103
)
  
642,096
 
Corporate debt securities
  
148,663
   
8
   
(1,059
)
  
147,612
 
Debt securities issued by U.S. government agencies
  
52,779
   
   
(168
)
  
52,611
 
Debt securities issued by the U.S. Treasury
  
1,409
   
   
(2
)
  
1,407
 
Debt securities issued by states of the U.S. and political subdivisions of the states
  
65,550
   
   
(740
)
  
64,810
 
Total securities with a maturity of more than one year
  
268,401
   
8
   
(1,969
)
  
266,440
 
Total available-for-sale securities
 
$
911,594
  
$
14
  
$
(3,072
)
 
$
908,536
 

     
Gross Unrealized
  
Estimated
 
December 31, 2016
 
Cost (1)
  
Gains
  
Losses
  
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
 

(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, 2017 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
  
476
  
$
551,446
  
$
(1,236
)
 
$
74,987
  
$
(575
)
 
$
626,433
  
$
(1,811
)
Debt securities issued by U.S. government agencies
  
45
   
107,788
   
(262
)
  
27,538
   
(118
)
  
135,326
   
(380
)
Debt securities issued by the U.S. Treasury
  
7
   
30,818
   
(19
)
  
   
   
30,818
   
(19
)
Debt securities issued by states of the U.S. and political subdivisions of the states
  
60
   
62,519
   
(545
)
  
24,572
   
(317
)
  
87,091
   
(862
)
Total temporarily impaired securities
  
588
  
$
752,571
  
$
(2,062
)
 
$
127,097
  
$
(1,010
)
 
$
879,668
  
$
(3,072
)