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Fair Value - Carrying Amount, Estimated Fair Value and Level of Fair Value Hierarchy of Company's Financial Assets and Liabilities (Detail) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Estimate Fair Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Other invested assets, primarily mortgage loans $ 731 $ 594
Short term debt 193 107
Long term debt 11,532 10,796
Carrying Amount [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Other invested assets, primarily mortgage loans 722 591
Short term debt 191 107
Long term debt 11,222 10,655
Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short term debt 152 104
Long term debt 10,316 10,150
Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Other invested assets, primarily mortgage loans 731 594
Short term debt 41 3
Long term debt $ 1,216 $ 646