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Investments (Tables)
 6 Months Ended
Jun. 30, 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 June 30, 2017:

One year or less
69%
After one year but within two years
20%
After two years but within three and a half years
11%
Total
100%
Summary of Investments
The following is a summary of our investments (in thousands):

   
Gross Unrealized
   
June 30, 2017
 
Cost (1)
  
Gains
  
Losses
  
Estimated Fair Value
 
Available-for-sale securities:
            
Corporate debt securities (2)
 
$
369,416
  
$
27
  
$
(360
)
 
$
369,083
 
Debt securities issued by U.S. government agencies
  
63,125
   
—
   
(98
)
  
63,027
 
Debt securities issued by the U.S. Treasury (2)
  
28,295
   
—
   
(25
)
  
28,270
 
Debt securities issued by states of the U.S. and political subdivisions of the states (2)
  
43,528
   
12
   
(122
)
  
43,418
 
Other municipal debt securities
  
3,000
   
—
   
(2
)
  
2,998
 
Total securities with a maturity of one year or less
  
507,364
   
39
   
(607
)
  
506,796
 
Corporate debt securities
  
159,233
   
42
   
(674
)
  
158,601
 
Debt securities issued by U.S. government agencies
  
19,181
   
—
   
(108
)
  
19,073
 
Debt securities issued by the U.S. Treasury
  
—
   
—
   
—
   
—
 
Debt securities issued by states of the U.S. and political subdivisions of the states
  
49,957
   
—
   
(336
)
  
49,621
 
Total securities with a maturity of more than one year
  
228,371
   
42
   
(1,118
)
  
227,295
 
Total available-for-sale securities
 
$
735,735
  
$
81
  
$
(1,725
)
 
$
734,091
 

   
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
 

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

(2)
Includes investments classified as cash equivalents on our condensed consolidated balance sheet.
Temporarily Impaired Investments

Investments we consider to be temporarily impaired at June 30, 2017 were 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
  
354
  
$
423,899
  
$
(1,001
)
 
$
18,197
  
$
(33
)
 
$
442,096
  
$
(1,034
)
Debt securities issued by U.S. government agencies
  
42
   
81,100
   
(206
)
  
—
   
—
   
81,100
   
(206
)
Debt securities issued by the U.S. Treasury
  
4
   
25,772
   
(25
)
  
—
   
—
   
25,772
   
(25
)
Debt securities issued by states of the U.S. and political subdivisions of the states
  
74
   
83,078
   
(385
)
  
2,687
   
(73
)
  
85,765
   
(458
)
Other municipal debt securities
  
1
   
2,998
   
(2
)
  
—
   
—
   
2,998
   
(2
)
Total temporarily impaired securities
  
475
  
$
616,847
  
$
(1,619
)
 
$
20,884
  
$
(106
)
 
$
637,731
  
$
(1,725
)