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Fair values of financial instruments carried at fair value - Transfers between Level 1 and Level 2 fair values (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Available for sale    
Assets    
Transfers from Level 1 to Level 2 $ 2,231 $ 162
Transfers from Level 2 to Level 1 11,173 1,314
Trading assets    
Assets    
Transfers from Level 1 to Level 2 1,507 1,614
Transfers from Level 2 to Level 1 1,384 0
Financial assets designated at fair value    
Assets    
Transfers from Level 1 to Level 2 0 122
Transfers from Level 2 to Level 1 0 0
Derivatives    
Assets    
Transfers from Level 1 to Level 2 0 465
Transfers from Level 2 to Level 1 0 0
Trading liabilities    
Liabilities    
Transfers from Level 1 to Level 2 35 2,699
Transfers from Level 2 to Level 1 683 341
Financial liabilities designated at fair value    
Liabilities    
Transfers from Level 1 to Level 2 0 0
Transfers from Level 2 to Level 1 0 0
Derivatives    
Liabilities    
Transfers from Level 1 to Level 2 0 209
Transfers from Level 2 to Level 1 $ 0 $ 0