Changes in Accumulated Other Comprehensive Loss by Component, Net of Tax (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Accumulated Other Comprehensive Income Loss [Line Items]        
Beginning Balance     $ 5,049 $ 4,915
Other comprehensive income (loss) before reclassifications     (15) (1)
Amounts reclassified from accumulated other comprehensive loss     4 4
Net other comprehensive income (loss) $ (3) $ 1 (11) 3
Ending Balance 5,238 5,165 5,238 5,165
Employee Benefit Plan Funded Status        
Accumulated Other Comprehensive Income Loss [Line Items]        
Beginning Balance     (126) (116)
Ending Balance (126) (116) (126) (116)
Amortization of Prior Service Cost        
Accumulated Other Comprehensive Income Loss [Line Items]        
Beginning Balance     1 1
Ending Balance 1 1 1 1
Amortization of Actuarial Loss        
Accumulated Other Comprehensive Income Loss [Line Items]        
Beginning Balance     36 31
Amounts reclassified from accumulated other comprehensive loss     4 4
Net other comprehensive income (loss)     4 4
Ending Balance 40 35 40 35
Foreign Currency Translation        
Accumulated Other Comprehensive Income Loss [Line Items]        
Beginning Balance     2 3
Other comprehensive income (loss) before reclassifications       (1)
Net other comprehensive income (loss)       (1)
Ending Balance 2 2 2 2
Loss on Cash Flow Hedge        
Accumulated Other Comprehensive Income Loss [Line Items]        
Beginning Balance     (1) (1)
Other comprehensive income (loss) before reclassifications     (15)  
Net other comprehensive income (loss)     (15)  
Ending Balance (16) (1) (16) (1)
Accumulated Other Comprehensive Loss        
Accumulated Other Comprehensive Income Loss [Line Items]        
Beginning Balance     (88) (82)
Net other comprehensive income (loss)     (11) 3
Ending Balance $ (99) $ (79) $ (99) $ (79)