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Marketable Securities
 12 Months Ended
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]  
Marketable Securities
Marketable Securities
Our investments in marketable securities as of December 31, 2017 and 2016 consisted of the following:
 
2017
 
2016
(millions)
Amortized Cost
 
Fair Value
 
Amortized Cost
 
Fair Value
Corporate debt securities
$
68

 
$
68

 
$
69

 
$
69

U.S. government and agency debt securities
6

 
6

 
14

 
14

Asset-backed debt securities
11

 
11

 
2

 
2

Certificates of deposit
13

 
13

 
6

 
6

Municipal debt securities
1

 
1

 
—

 
—

Total marketable securities
$
99

 
$
99

 
$
91

 
$
91


The realized and unrealized gains and losses as of and for the years ended December 31, 2017, 2016 and 2015 were immaterial.
Contractual maturities of marketable securities as of December 31, 2017 were as follows:
(millions)
Amortized Cost
 
Fair Value
Due in 1 year or less
$
62

 
$
62

Due in 1-5 years
37

 
37

Total marketable securities
$
99

 
$
99


Actual maturities may differ from the contractual maturities because issuers of the securities may have the right to prepay them.