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Fair value measurement - Summary of Carrying Amount of Hedged Financial Instruments, Related Notional Amount of Hedge and Estimated Fair Value of Derivatives (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Securities available-for-sale    
Derivative [Line Items]    
Carrying amount $ 12,159 $ 9,184
Notional amount of hedge 12,064 9,233
Unrealized Gain 53 83
Unrealized (Loss) (354) (342)
Long-term debt    
Derivative [Line Items]    
Carrying amount 23,543 20,511
Notional amount of hedge 23,450 20,450
Unrealized Gain 336 330
Unrealized (Loss) $ (164) $ (203)