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Fair Value Measurements (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2015
Financial Instruments        
Cash and cash equivalents $ 791 $ 900 $ 899 $ 985
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 6.55% 6.93%    
Fair Value | Level 1        
Financial Instruments        
Cash and cash equivalents $ 791 $ 900    
Fair Value | Level 2 | Retail        
Financial Instruments        
Long-term debt 1,815 1,741    
Fair Value | Level 2 | Institutional        
Financial Instruments        
Long-term debt 552 532    
Fair Value | Level 2 | Other        
Financial Instruments        
Long-term debt 201 207    
Book Value        
Financial Instruments        
Cash and cash equivalents 791 900    
Book Value | Retail        
Financial Instruments        
Long-term debt 1,753 1,753    
Book Value | Institutional        
Financial Instruments        
Long-term debt 533 533    
Book Value | Other        
Financial Instruments        
Long-term debt $ 202 $ 208