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Goodwill and Other Intangibles - Narrative (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]        
Goodwill decrease due to the sale of a subsidiary $ (2)     $ (2)
Amortizable intangible assets related to property and casualty insurance acquisitions   $ 26 $ 34  
Accumulated amortization   30 25  
Amortization of intangible assets   $ 8 $ 8 $ 8
Weighted average useful life of finite-lived intangible assets acquired   3 years    
Future amortization of intangibles in next year   $ 8    
Future amortization of intangibles in year two   8    
Future amortization of intangibles in year three   8    
Future amortization of intangibles in year four   $ 2