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Investments (Tables)
 9 Months Ended
Sep. 30, 2018
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 September 30, 2018:

One year or less
80%
After one year but within two years
15%
After two years but within three years
5%
Total
100%
Summary of Investments
The following is a summary of our investments (in thousands):

   
Gross Unrealized
   
September 30, 2018
 
Cost (1)
  
Gains
  
Losses
  
Estimated Fair Value
 
Available-for-sale securities:
            
Corporate debt securities (2)
 
$
841,397
  
$
23
  
$
(1,264
)
 
$
840,156
 
Debt securities issued by U.S. government agencies
  
143,008
   
2
   
(169
)
  
142,841
 
Debt securities issued by the U.S. Treasury (2)
  
154,280
   
—
   
(89
)
  
154,191
 
Debt securities issued by states of the U.S. and political subdivisions of the states (2)
  
49,873
   
—
   
(330
)
  
49,543
 
Total securities with a maturity of one year or less
  
1,188,558
   
25
   
(1,852
)
  
1,186,731
 
Corporate debt securities
  
240,208
   
16
   
(1,420
)
  
238,804
 
Debt securities issued by U.S. government agencies
  
32,435
   
—
   
(109
)
  
32,326
 
Debt securities issued by the U.S. Treasury
  
9,872
   
—
   
(10
)
  
9,862
 
Debt securities issued by states of the U.S. and political subdivisions of the states
  
31,666
   
—
   
(551
)
  
31,115
 
Total securities with a maturity of more than one year
  
314,181
   
16
   
(2,090
)
  
312,107
 
Total available-for-sale securities
 
$
1,502,739
  
$
41
  
$
(3,942
)
 
$
1,498,838
 

   
Gross Unrealized
   
December 31, 2017
 
Cost (1)
  
Gains
  
Losses
  
Estimated Fair Value
 
Available-for-sale securities:
            
Corporate debt securities
 
$
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
 

(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
The following is a summary of our investments we consider to be temporarily impaired at September 30, 2018. We believe that the decline in value of these securities is temporary and is primarily related to the change in market interest rates since purchase. We believe it is more likely than not that we will be able to hold our debt securities to maturity. Therefore, we anticipate full recovery of our debt securities’ amortized cost basis at maturity.

     
Less than 12 Months of
Temporary Impairment
  
More than 12 Months of
Temporary Impairment
  
Total Temporary
Impairment
 
 (In thousands)
 
Number of
Investments
  
Estimated
Fair Value
  
Unrealized
Losses
  
Estimated
Fair Value
  
Unrealized
Losses
  
Estimated
Fair Value
  
Unrealized
Losses
 
Corporate debt securities
  
495
  
$
889,172
  
$
(1,687
)
 
$
83,706
  
$
(997
)
 
$
972,878
  
$
(2,684
)
Debt securities issued by U.S. government agencies
  
51
   
132,231
   
(139
)
  
26,862
   
(139
)
  
159,093
   
(278
)
Debt securities issued by the U.S. Treasury
  
32
   
164,053
   
(99
)
  
—
   
—
   
164,053
   
(99
)
Debt securities issued by states of the U.S. and political subdivisions of the states
  
48
   
25,687
   
(191
)
  
51,747
   
(690
)
  
77,434
   
(881
)
Total temporarily impaired securities
  
626
  
$
1,211,143
  
$
(2,116
)
 
$
162,315
  
$
(1,826
)
 
$
1,373,458
  
$
(3,942
)