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Fair Value Measurements (Details) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Dec. 31, 2015
Financial Instruments        
Cash and cash equivalents $ 784 $ 900 $ 985 $ 985
Short-term investments $ 100 $ 0    
Institutional and Other | Minimum        
Financial Instruments        
Fair value assumption, interest rate 0.00% 0.00%    
Institutional and Other | Maximum        
Financial Instruments        
Fair value assumption, interest rate 6.63% 6.93%    
Fair Value | Level 1        
Financial Instruments        
Cash and cash equivalents $ 784 $ 900    
Short-term investments 100 0    
Fair Value | Level 2 | Retail        
Financial Instruments        
Long-term debt 1,825 1,741    
Fair Value | Level 2 | Institutional        
Financial Instruments        
Long-term debt 572 532    
Fair Value | Level 2 | Other        
Financial Instruments        
Long-term debt 198 207    
Book Value        
Financial Instruments        
Cash and cash equivalents 784 900    
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 $ 199 $ 208