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) |