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Investments (Tables)
 3 Months Ended
Mar. 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 March 31, 2018:

One year or less
77%
After one year but within two years
18%
After two years but within three and a half years
5%
Total
100%
Summary of Investments
The following is a summary of our investments (in thousands):

   
Gross Unrealized
   
March 31, 2018
 
Cost (1)
  
Gains
  
Losses
  
Estimated Fair Value
 
Available-for-sale securities:
            
Corporate debt securities (2)
 
$
433,297
  
$
1
  
$
(1,286
)
 
$
432,012
 
Debt securities issued by U.S. government agencies
  
129,235
   
—
   
(323
)
  
128,912
 
Debt securities issued by the U.S. Treasury (2)
  
54,076
   
1
   
(43
)
  
54,034
 
Debt securities issued by states of the U.S. and political subdivisions of the states (2)
  
31,624
   
1
   
(194
)
  
31,431
 
Total securities with a maturity of one year or less
  
648,232
   
3
   
(1,846
)
  
646,389
 
Corporate debt securities
  
126,128
   
5
   
(1,807
)
  
124,326
 
Debt securities issued by U.S. government agencies
  
21,547
   
—
   
(162
)
  
21,385
 
Debt securities issued by states of the U.S. and political subdivisions of the states
  
52,746
   
—
   
(785
)
  
51,961
 
Total securities with a maturity of more than one year
  
200,421
   
5
   
(2,754
)
  
197,672
 
Total available-for-sale securities
 
$
848,653
  
$
8
  
$
(4,600
)
 
$
844,061
 

   
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
Investments we consider to be temporarily impaired at March 31, 2018 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
  
420
  
$
470,960
  
$
(2,308
)
 
$
71,370
  
$
(785
)
 
$
542,330
  
$
(3,093
)
Debt securities issued by U.S. government agencies
  
52
   
124,729
   
(397
)
  
25,569
   
(88
)
  
150,298
   
(485
)
Debt securities issued by the U.S. Treasury
  
7
   
32,329
   
(43
)
  
—
   
—
   
32,329
   
(43
)
Debt securities issued by states of the U.S. and political subdivisions of the states
  
50
   
48,477
   
(588
)
  
32,145
   
(391
)
  
80,622
   
(979
)
Total temporarily impaired securities
  
529
  
$
676,495
  
$
(3,336
)
 
$
129,084
  
$
(1,264
)
 
$
805,579
  
$
(4,600
)