Derivative instruments - Liquidity Risk and Credit-Related Contingent Features (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregate fair value of net derivative payables	$ 24,822	$ 32,303
Collateral posted	22,858	27,585
Single-notch downgrade
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of additional collateral to be posted upon downgrade	895	1,046
Amount required to settle contracts with termination triggers upon downgrade	287	366
Two-notch downgrade
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of additional collateral to be posted upon downgrade	3,164	3,331
Amount required to settle contracts with termination triggers upon downgrade	$ 1,141	$ 1,388