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Investments - Impairment and credit losses on fixed income securities (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Investments [Abstract]      
Net unrealized gains related to changes in valuation of fixed income securities subsequent to impairment measurement date $ 113 $ 131  
Other-than-temporary impairment losses included in accumulated other comprehensive income      
Amount of other-than-temporary impairment losses included in accumulated other comprehensive income for fixed income securities, not included in earnings (53) (71)  
Credit Losses on Fixed Income Securities      
Beginning balance (176) (200) $ (209)
Additional credit loss for securities previously other-than-temporarily impaired (9) (16) (8)
Additional credit loss for securities not previously other-than-temporarily impaired (8) (19) (18)
Reduction in credit loss for securities disposed or collected 54 58 33
Change in credit loss due to accretion of increase in cash flows 1 1 2
Ending balance (138) (176) $ (200)
Municipal      
Other-than-temporary impairment losses included in accumulated other comprehensive income      
Amount of other-than-temporary impairment losses included in accumulated other comprehensive income for fixed income securities, not included in earnings (4) (5)  
Corporate      
Other-than-temporary impairment losses included in accumulated other comprehensive income      
Amount of other-than-temporary impairment losses included in accumulated other comprehensive income for fixed income securities, not included in earnings 0 (5)  
ABS      
Other-than-temporary impairment losses included in accumulated other comprehensive income      
Amount of other-than-temporary impairment losses included in accumulated other comprehensive income for fixed income securities, not included in earnings (8) (11)  
RMBS      
Other-than-temporary impairment losses included in accumulated other comprehensive income      
Amount of other-than-temporary impairment losses included in accumulated other comprehensive income for fixed income securities, not included in earnings (37) (43)  
CMBS      
Other-than-temporary impairment losses included in accumulated other comprehensive income      
Amount of other-than-temporary impairment losses included in accumulated other comprehensive income for fixed income securities, not included in earnings $ (4) $ (7)