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Fair Value Measurements (Tables)
 12 Months Ended
Dec. 31, 2019
Fair Value Disclosures [Abstract]  
Fair Value, Measurement Inputs, Disclosure [Table Text Block]
Assets and liabilities measured at fair value on a recurring basis as of December 31, 2019 and 2018, are summarized in the following tables. 
Dollar amounts in millions
December 31,
2019
 
Quoted Prices in
Active Markets
for Identical
Assets
(Level 1)
 
Significant
Other Observable
Inputs
(Level 2)
 
Significant
Unobservable
Inputs
(Level 3)
Available for sale securities
$
5

 
$
—

 
$
—

 
$
5

Trading securities
4

 
4

 
—

 
—

 
Dollar amounts in millions
December 31,
2018
 
Quoted Prices in
Active Markets
for Identical
Assets
(Level 1)
 
Significant
Other Observable
Inputs
(Level 2)
 
Significant
Unobservable
Inputs
(Level 3)
Available for sale securities
$
6

 
$
—

 
$
—

 
$
6

Trading securities
3

 
3

 
—

 
—
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]
The following table summarizes changes in assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) during the twelve months ended December 31, 2019 and 2018. 
Dollar amounts in millions
Available for
sale securities
Balance at December 31, 2017
$
6

Return of principal on ARS
—

Total realized/unrealized gains
—

Balance at December 31, 2018
6

Return of principal on ARS
(1
)
Total realized/unrealized gains
—

Balance at December 31, 2019
$
5