Fair values of financial instruments carried at fair value - Quantitative information about significant unobservable inputs in Level 3 valuations (Details) - Recurring fair value measurement $ in Millions	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 21,463	$ 19,809
Financial liabilities	8,838	7,371
Changes in FV due to refinement in application of levelling methodology		2,900
Level 3 | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	3	4
Level 3 | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	5,039	5,063
Level 3 | – equity-linked notes one
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	3,988	3,768
Level 3 | – FX-linked notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	579	1,046
Level 3 | – other
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	472	249
Level 3 | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	3,717	2,302
Level 3 | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	79	2
Level 3 | – other
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	16	2
Level 3 | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	10,448	9,551
Changes in FV due to refinement in application of levelling methodology		1,500
Level 3 | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,867	1,836
Level 3 | – CLO/CDO
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	113	373
Level 3 | – other ABSs
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,754	1,463
Level 3 | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1	40
Level 3 | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3	3
Level 3 | – other
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3	3
Level 3 | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	75	66
Level 3 | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3,771	2,070
Level 3 | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	5,298	6,243
Changes in FV due to refinement in application of levelling methodology		1,400
Level 3 | – structured certificates
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,488	1,515
Level 3 | – repurchase agreements
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	778	1,604
Financial liabilities	63
Level 3 | – other
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3,032	3,124
Level 3 | Interest rate | securitisation swaps
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	869	640
Level 3 | Interest rate | long-dated swaptions
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	735	51
Level 3 | Interest rate | other
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	335	155
Level 3 | Interest rate | securitisation swaps
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	283	314
Level 3 | Interest rate | long-dated swaptions
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,778	838
Level 3 | Interest rate | other
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	412	255
Level 3 | Foreign exchange | other
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	139	104
Level 3 | Foreign exchange | FX options
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	191	218
Level 3 | Foreign exchange | other
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	142	119
Level 3 | Foreign exchange | FX options
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	109	93
Level 3 | Equities | other
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	551	712
Level 3 | Equities | long-dated single stock options
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	821	293
Level 3 | Equities | other
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	192	78
Level 3 | Equities | long-dated single stock options
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	750	230
Level 3 | Credit | other
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	76	129
Level 3 | Credit | other
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 105	$ 143
Prepayment rate | Market proxy | Bottom of range | – CLO/CDO
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0.00	0.00
Prepayment rate | Market proxy | Top of range | – CLO/CDO
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0.09	0.09
Prepayment rate | Model – Discounted cash flow | Bottom of range | Interest rate | securitisation swaps
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, liabilities, percentage	0.06	0.06
Prepayment rate | Model – Discounted cash flow | Bottom of range | Interest rate | securitisation swaps
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0.06	0.06
Prepayment rate | Model – Discounted cash flow | Top of range | Interest rate | securitisation swaps
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, liabilities, percentage	0.07	0.07
Prepayment rate | Model – Discounted cash flow | Top of range | Interest rate | securitisation swaps
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0.07	0.07
Bid quotes | Market proxy | Bottom of range | – CLO/CDO
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0	0
Bid quotes | Market proxy | Bottom of range | – other ABSs
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0	0
Bid quotes | Market proxy | Top of range | – CLO/CDO
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	99	100
Bid quotes | Market proxy | Top of range | – other ABSs
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	100	101
Equity volatility | Model – Option model | Bottom of range | – equity-linked notes one
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, liabilities, percentage	0.06	0.05
Equity volatility | Model – Option model | Bottom of range | Equities | long-dated single stock options
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, liabilities, percentage	0.00	0.00
Equity volatility | Model – Option model | Bottom of range | Equities | long-dated single stock options
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0.00	0.00
Equity volatility | Model – Option model | Top of range | – equity-linked notes one
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, liabilities, percentage	1.61	0.90
Equity volatility | Model – Option model | Top of range | Equities | long-dated single stock options
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, liabilities, percentage	1.31	0.89
Equity volatility | Model – Option model | Top of range | Equities | long-dated single stock options
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	1.31	0.89
Equity correlation | Model – Option model | Bottom of range | – equity-linked notes two
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, liabilities, percentage	0.22	0.09
Equity correlation | Model – Option model | Top of range | – equity-linked notes two
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, liabilities, percentage	0.92	0.93
FX volatility | Model – Option model | Bottom of range | – FX-linked notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, liabilities, percentage	0.01	0.01
FX volatility | Model – Option model | Bottom of range | Foreign exchange | FX options
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, liabilities, percentage	0.01	0.01
FX volatility | Model – Option model | Bottom of range | Foreign exchange | FX options
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0.01	0.01
FX volatility | Model – Option model | Top of range | – FX-linked notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, liabilities, percentage	0.34	0.23
FX volatility | Model – Option model | Top of range | Foreign exchange | FX options
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, liabilities, percentage	0.49	0.25
FX volatility | Model – Option model | Top of range | Foreign exchange | FX options
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0.49	0.25
Credit spread | Model – Discounted cash flow | Bottom of range | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0.016	0.004
Credit spread | Model – Discounted cash flow | Top of range | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0.021	0.02
IR volatility | Model – Option model | Bottom of range | Interest rate | long-dated swaptions
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, liabilities, percentage	0.07	0.08
IR volatility | Model – Option model | Bottom of range | Interest rate | long-dated swaptions
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0.07	0.08
IR volatility | Model – Option model | Top of range | Interest rate | long-dated swaptions
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, liabilities, percentage	0.33	0.22
IR volatility | Model – Option model | Top of range | Interest rate | long-dated swaptions
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0.33	0.22
Credit volatility | Model – Discounted cash flow | Bottom of range | – structured certificates
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0.11	0.04
Credit volatility | Model – Discounted cash flow | Top of range | – structured certificates
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Significant unobservable input, assets, percentage	0.11	0.04
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 41,785	$ 43,627
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	10,404	9,476	$ 8,245	$ 7,080
Changes in FV due to refinement in application of levelling methodology		2,900
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Level 3 | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	9,756	8,831
Changes in FV due to refinement in application of levelling methodology		1,700
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Level 3 | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	98	28
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Level 3 | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	39
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Level 3 | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 0	$ 0