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Available-for-sale investments (Unrealized Losses and Fair Values) (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, Fair value $ 14,442 $ 18,281
Less than 12 months, Unrealized losses (67) (250)
12 months or greater, Fair value 667 493
12 months or greater, Unrealized losses (8) (8)
U.S. Treasury securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, Fair value 6,242 5,774
Less than 12 months, Unrealized losses (17) (68)
12 months or greater, Fair value 5 0
12 months or greater, Unrealized losses 0 0
Other government-related debt securities - U.S. [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, Fair value 118 201
Less than 12 months, Unrealized losses (1) (3)
12 months or greater, Fair value 12 0
12 months or greater, Unrealized losses 0 0
Other government-related debt securities - Foreign and other [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, Fair value 515 1,192
Less than 12 months, Unrealized losses (2) (34)
12 months or greater, Fair value 65 17
12 months or greater, Unrealized losses (2) 0
Corporate debt securities - Financial [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, Fair value 1,997 3,975
Less than 12 months, Unrealized losses (6) (37)
12 months or greater, Fair value 151 44
12 months or greater, Unrealized losses (1) 0
Corporate debt securities - Industrial [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, Fair value 2,434 3,913
Less than 12 months, Unrealized losses (14) (61)
12 months or greater, Fair value 276 149
12 months or greater, Unrealized losses (3) (2)
Corporate debt securities - Other [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, Fair value 326 486
Less than 12 months, Unrealized losses (2) (7)
12 months or greater, Fair value 26 7
12 months or greater, Unrealized losses 0 0
Residential mortgage-backed securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, Fair value 1,723 1,631
Less than 12 months, Unrealized losses (8) (26)
12 months or greater, Fair value 94 158
12 months or greater, Unrealized losses (2) (3)
Other mortgage- and asset-backed securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, Fair value 1,074 1,087
Less than 12 months, Unrealized losses (4) (10)
12 months or greater, Fair value 38 118
12 months or greater, Unrealized losses 0 (3)
Equity Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, Fair value 13 22
Less than 12 months, Unrealized losses (13) (4)
12 months or greater, Fair value 0 0
12 months or greater, Unrealized losses $ 0 $ 0