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Investments (Unrealized Losses and Fair Values) (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair values $ 13,974 $ 25,250
Less than 12 months, Unrealized losses (447) (191)
12 months or more, Fair values 1,226 2,061
12 months or more, Unrealized losses (47) (31)
U.S. Treasury securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair values 3,355 7,728
Less than 12 months, Unrealized losses (85) (70)
12 months or more, Fair values 36 195
12 months or more, Unrealized losses (1) (2)
Other government-related debt securities - U.S. [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair values 102 188
Less than 12 months, Unrealized losses (3) (1)
12 months or more, Fair values 26 34
12 months or more, Unrealized losses (1) (1)
Other government-related debt securities - Foreign and other [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair values 1,209 1,163
Less than 12 months, Unrealized losses (56) (9)
12 months or more, Fair values 112 115
12 months or more, Unrealized losses (6) (2)
Corporate debt securities - Financial [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair values 3,673 5,928
Less than 12 months, Unrealized losses (115) (28)
12 months or more, Fair values 258 462
12 months or more, Unrealized losses (9) (6)
Corporate debt securities - Industrial [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair values 3,244 5,760
Less than 12 months, Unrealized losses (110) (43)
12 months or more, Fair values 314 612
12 months or more, Unrealized losses (12) (9)
Corporate debt securities - Other [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair values 584 868
Less than 12 months, Unrealized losses (20) (4)
12 months or more, Fair values 65 117
12 months or more, Unrealized losses (3) (2)
Residential mortgage-backed securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair values 1,267 1,838
Less than 12 months, Unrealized losses (42) (24)
12 months or more, Fair values 303 276
12 months or more, Unrealized losses (11) (6)
Other mortgage- and asset-backed securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair values 540 1,777
Less than 12 months, Unrealized losses (16) (12)
12 months or more, Fair values 112 250
12 months or more, Unrealized losses $ (4) $ (3)