Commercial Paper and Long-term Debt (Tables)
 12 Months Ended
Dec. 31, 2018
Debt Disclosure [Abstract]  
Schedule of Long-term Debt Long-term debt consisted of the following at December 31: 
(In millions, except percentages)
2018

 
2017

$500 notes due 2020, 4.40%
$
499

 
$
499

$1,000 notes due 2020, 3.125%
998

 
996

$1,100 notes due 2022, 2.50%
1,096

 
1,095

$300 notes due 2024, 3.15%
298

 
297

$382 notes due 2027, 7.20%
373

 
372

$185 notes due 2028, 7.00%
185

 
185

$600 notes due 2040, 4.875%
592

 
592

$425 notes due 2041, 4.70%
419

 
419

$300 notes due 2044, 4.20%
295

 
295

Total debt issued and outstanding
$
4,755

 
$
4,750
Fair Value of Long-term Debt The estimated fair value of long-term debt was the following at December 31:
(In millions)
2018

 
2017

Fair value of long-term debt
$
5,063

 
$
5,293
Schedule of Long-term Debt Instruments The adjustments to the principal amounts of long-term debt were as follows at December 31: 
(In millions)
2018

 
2017

Principal
$
4,792

 
$
4,792

Unamortized issue discounts
(30
)
 
(34
)
Unamortized interest rate lock costs
(7
)
 
(8
)
Total
$
4,755

 
$
4,750


 
The aggregate amounts of principal payments due on long-term debt for the next five years are:
(In millions)
 
2019
$
—

2020
1,500

2021
—

2022
1,100

2023
—

Thereafter
2,192