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Investments (Tables)
 12 Months Ended
Dec. 31, 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 December 31, 2018:

One year or less
  
77
%
After one year but within two years
  
20
%
After two years but within three and one half years
  
3
%
Total
  
100
%
Summary of Investments
The following is a summary of our investments (in thousands):

December 31, 2018
 
Cost (1)
  
Gains
  
Losses
  
Fair Value
 
Available-for-sale securities:
            
Corporate debt securities (2)
 
$
956,879
  
$
13
  
$
(1,858
)
 
$
955,034
 
Debt securities issued by U.S. government agencies
  
168,839
   
3
   
(104
)
  
168,738
 
Debt securities issued by the U.S. Treasury (2)
  
244,640
   
15
   
(77
)
  
244,578
 
Debt securities issued by states of the U.S. and political subdivisions of the states
  
63,572
   
—
   
(323
)
  
63,249
 
Total securities with a maturity of one year or less
  
1,433,930
   
31
   
(2,362
)
  
1,431,599
 
Corporate debt securities
  
299,018
   
194
   
(1,286
)
  
297,926
 
Debt securities issued by U.S. government agencies
  
107,789
   
194
   
(109
)
  
107,874
 
Debt securities issued by the U.S. Treasury
  
15,600
   
—
   
(24
)
  
15,576
 
Debt securities issued by states of the U.S. and political subdivisions of the states
  
16,980
   
—
   
(287
)
  
16,693
 
Total securities with a maturity of more than one year
  
439,387
   
388
   
(1,706
)
  
438,069
 
Total available-for-sale securities
 
$
1,873,317
  
$
419
  
$
(4,068
)
 
$
1,869,668
 
Equity securities:
                
Total equity securities included in other current assets (3)
 
$
1,212
   
137
   
—
   
1,349
 
Total available-for-sale and equity securities
 
$
1,874,529
  
$
556
  
$
(4,068
)
 
$
1,871,017
 

     
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
 

(1)
We hold our available-for-sale securities at amortized cost.

(2)
Includes investments classified as cash equivalents on our consolidated balance sheet.

(3)
We recognize our equity securities at cost minus impairments, plus or minus changes resulting from observable price changes in orderly transactions for the identical or similar investment of the same issuer on our consolidated balance sheet.
Temporarily Impaired Investments
Investments we consider to be temporarily impaired at December 31, 2018 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
  
546
  
$
1,000,461
  
$
(1,936
)
 
$
126,357
  
$
(1,208
)
 
$
1,126,818
  
$
(3,144
)
Debt securities issued by U.S. government agencies
  
50
   
161,312
   
(109
)
  
34,403
   
(104
)
  
195,715
   
(213
)
Debt securities issued by the U.S. Treasury
  
36
   
183,212
   
(100
)
  
1,413
   
(1
)
  
184,625
   
(101
)
Debt securities issued by states of the U.S. and political subdivisions of the states
  
49
   
13,868
   
(14
)
  
62,883
   
(596
)
  
76,751
   
(610
)
Total temporarily impaired securities
  
681
  
$
1,358,853
  
$
(2,159
)
 
$
225,056
  
$
(1,909
)
 
$
1,583,909
  
$
(4,068
)