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Fair Value of Financial Instruments (Tables)
 12 Months Ended
Dec. 31, 2017
Fair Value Disclosures [Abstract]  
Fair value of assets and liabilities measured on a recurring basis
The following table presents the financial assets and liabilities we record at fair value on a recurring basis identified by the level of inputs used to determine fair value. See Note 1 for the definitions of these levels.
 
 
December 31, 2017
 
December 31, 2016
$ in millions
 
Level 1
 
Level 2
 
Total
 
Level 1
 
Level 2
 
Total
Financial Assets (Liabilities)
 
 
 
 
 
 
 
 
 
 
 
 
Marketable securities
 
 
 
 
 
 
 
 
 
 
 
 
Trading
 
$
342

 
$
1

 
$
343

 
$
321

 
$
2

 
$
323

Available-for-sale
 
10

 
—

 
10

 
7

 
—

 
7

Derivatives
 
—

 
—

 
—

 
—

 
8

 
8