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Investments (Tables)
 12 Months Ended
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]  
Investments
The following tables summarize our investments as of the dates indicated:
 
December 31, 2017
 
Amortized
 
Gross
Unrealized
 
Estimated
 
Cost
 
Gains
 
Losses
 
Fair Value
 
(In millions)
Corporate debt securities
$
1,591

 
$
1

 
$
4

 
$
1,588

U.S. Treasury notes
389

 
—

 
1

 
388

GSEs
255

 
—

 
2

 
253

Municipal securities
142

 
—

 
1

 
141

Asset-backed securities
117

 
—

 
—

 
117

Certificates of deposit
37

 
—

 
—

 
37

Subtotal - current investments
2,531

 
1

 
8

 
2,524

Corporate debt securities
101

 
—

 
—

 
101

U.S. treasury notes
68

 
—

 
—

 
68

Subtotal - restricted investments, current
169

 
—

 
—

 
169

 
$
2,700

 
$
1

 
$
8

 
$
2,693

 
December 31, 2016
 
Amortized Cost
 
Gross
Unrealized
 
Estimated Fair Value
 
 
Gains
 
Losses
 
 
(In millions)
Corporate debt securities
$
1,180

 
$
1

 
$
2

 
$
1,179

U.S. treasury notes
84

 
—

 
—

 
84

GSEs
232

 
—

 
1

 
231

Municipal securities
143

 
—

 
1

 
142

Asset-backed securities
69

 
—

 
—

 
69

Certificates of deposit
53

 
—

 
—

 
53

 
$
1,761

 
$
1

 
$
4

 
$
1,758
Contractual maturities of investments
The contractual maturities of our available-for-sale investments as of December 31, 2017 are summarized below:
 
Amortized
Cost
 
Estimated
Fair Value
 
(In millions)
Due in one year or less
$
1,722

 
$
1,721

Due after one year through five years
972

 
966

Due after five years through ten years
6

 
6

 
$
2,700

 
$
2,693
Available-for-sale investments
The following table segregates those available-for-sale investments that have been in a continuous loss position for less than 12 months, and those that have been in a continuous loss position for 12 months or more as of December 31, 2017.
 
In a Continuous Loss Position
for Less than 12 Months
 
In a Continuous Loss Position
for 12 Months or More
 
Estimated
Fair
Value
 
Unrealized
Losses
 
Total Number of Positions
 
Estimated
Fair
Value
 
Unrealized
Losses
 
Total Number of Positions
 
(Dollars in millions)
Corporate debt securities
$
1,297

 
$
3

 
561

 
$
94

 
$
1

 
69

U.S. Treasury notes
470

 
1

 
89

 
—

 
—

 
—

GSEs
173

 
1

 
69

 
95

 
1

 
47

Municipal securities
—

 
—

 
—

 
38

 
1

 
48

 
$
1,940

 
$
5

 
719

 
$
227

 
$
3

 
164

The following table segregates those available-for-sale investments that have been in a continuous loss position for less than 12 months, and those that have been in a continuous loss position for 12 months or more as of December 31, 2016.
 
In a Continuous Loss Position
for Less than 12 Months
 
In a Continuous Loss Position
for 12 Months or More
 
Estimated
Fair
Value
 
Unrealized
Losses
 
Total Number of Positions
 
Estimated
Fair
Value
 
Unrealized
Losses
 
Total Number of Positions
 
(Dollars in millions)
Corporate debt securities
$
542

 
$
2

 
378

 
$
—

 
$
—

 
—

GSEs
198

 
1

 
73

 
—

 
—

 
—

Municipal securities
101

 
1

 
129

 
—

 
—

 
—

 
$
841

 
$
4

 
580

 
$
—

 
$
—

 
—