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Income Taxes, Narrative (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Other income tax disclosures      
Effect of unrecognized tax benefit on income tax expense $ 37 $ 28 $ 26
Interest expense and penalties related to unrecognized income tax benefits 3 (1) $ 1
Net accrued interest and penalties, total $ 19 $ 15  
Federal income tax rate 35.00% 35.00% 35.00%
Future tax year      
Other income tax disclosures      
Federal income tax rate 21.00%    
Federal      
Operating loss carryforwards      
Deferred income tax asset for Federal NOL carryforwards (entities not on Federal return) $ 14    
Other income tax disclosures      
All audit periods prior to this year are closed 2013    
Federal | Minimum      
Operating loss carryforwards      
Expiration of NOL carryforwards Dec. 31, 2018    
Federal | Maximum      
Operating loss carryforwards      
Expiration of NOL carryforwards Dec. 31, 2037    
State      
Operating loss carryforwards      
NOL carryforwards $ 2,823    
Deferred income tax asset for State NOL carryforwards $ 153    
Other income tax disclosures      
All audit periods prior to this year are closed 2013    
State | Minimum      
Operating loss carryforwards      
Expiration of NOL carryforwards Dec. 31, 2018    
State | Maximum      
Operating loss carryforwards      
Expiration of NOL carryforwards Dec. 31, 2037