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Fair Value Measurements (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Financial Instruments      
Cash and cash equivalents $ 619 $ 900 $ 985
Short-term investments $ 100 $ 0  
Institutional and Other | Minimum      
Financial Instruments      
Fair value assumption, interest rate 4.74% 0.00%  
Institutional and Other | Maximum      
Financial Instruments      
Fair value assumption, interest rate 7.13% 6.93%  
Fair Value | Level 1      
Financial Instruments      
Cash and cash equivalents $ 619 $ 900  
Short-term investments 100 0  
Fair Value | Level 2 | Retail      
Financial Instruments      
Long-term debt 1,783 1,741  
Fair Value | Level 2 | Institutional      
Financial Instruments      
Long-term debt 522 532  
Fair Value | Level 2 | Other      
Financial Instruments      
Long-term debt 194 207  
Book Value      
Financial Instruments      
Cash and cash equivalents 619 900  
Short-term investments 100 0  
Book Value | Retail      
Financial Instruments      
Long-term debt 1,753 1,753  
Book Value | Institutional      
Financial Instruments      
Long-term debt 534 533  
Book Value | Other      
Financial Instruments      
Long-term debt $ 194 $ 208