Fair values of financial instruments carried at fair value - Financial instruments measured at fair value using a valuation technique with significant unobservable inputs - Level 3 (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Financial investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 404,328	$ 357,577
Changes in FV due to refinement in application of levelling methodology		59
Designated and otherwise mandatorily measured at fair value through profit or loss | Designated at fair value
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Changes in FV due to refinement in application of levelling methodology		86
Recurring fair value measurement | Trading liabilities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	79,612	83,170
Recurring fair value measurement | Designated at fair value
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	156,608	164,466
Recurring fair value measurement | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	303,059	239,497
Recurring fair value measurement | Financial investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	404,328	357,577
Recurring fair value measurement | Trading assets
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	208,964	254,271
Recurring fair value measurement | 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
Recurring fair value measurement | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	313,781	242,995
Recurring fair value measurement | 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
Recurring fair value measurement | Level 3 | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	3	4
Recurring fair value measurement | Level 3 | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	5,039	5,063
Recurring fair value measurement | Level 3 | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	3,717	2,302
Recurring fair value measurement | Level 3 | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	79	2
Recurring fair value measurement | Level 3 | Trading liabilities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	105	53	$ 49	$ 58
Recurring fair value measurement | Level 3 | Trading liabilities | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	3	4
Recurring fair value measurement | Level 3 | Trading liabilities | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Trading liabilities | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Trading liabilities | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	36	47
Recurring fair value measurement | Level 3 | Trading liabilities | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Trading liabilities | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Trading liabilities | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	66	2
Recurring fair value measurement | Level 3 | Designated at fair value
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	5,008	5,016	5,396	5,328
Recurring fair value measurement | Level 3 | Designated at fair value | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Designated at fair value | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Designated at fair value | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Designated at fair value | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	5,003	5,016
Recurring fair value measurement | Level 3 | Designated at fair value | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Designated at fair value | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Designated at fair value | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	5	0
Recurring fair value measurement | Level 3 | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	3,725	2,302	2,033	1,756
Recurring fair value measurement | Level 3 | Derivatives | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Derivatives | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Derivatives | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Derivatives | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Derivatives | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	0
Recurring fair value measurement | Level 3 | Derivatives | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	3,717	2,302
Recurring fair value measurement | Level 3 | Derivatives | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	8	0
Recurring fair value measurement | 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
Recurring fair value measurement | 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
Recurring fair value measurement | Level 3 | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,867	1,836
Recurring fair value measurement | Level 3 | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1	40
Recurring fair value measurement | Level 3 | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3	3
Recurring fair value measurement | Level 3 | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	75	66
Recurring fair value measurement | Level 3 | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3,771	2,070
Recurring fair value measurement | Level 3 | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	5,298	6,243
Recurring fair value measurement | Level 3 | Financial investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3,339	3,218	3,296	2,796
Changes in FV due to refinement in application of levelling methodology		1,200
Recurring fair value measurement | Level 3 | Financial investments | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	689	716
Recurring fair value measurement | Level 3 | Financial investments | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,095	874
Recurring fair value measurement | Level 3 | Financial investments | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Recurring fair value measurement | Level 3 | Financial investments | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Recurring fair value measurement | Level 3 | Financial investments | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Recurring fair value measurement | Level 3 | Financial investments | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Recurring fair value measurement | Level 3 | Financial investments | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,555	1,628
Recurring fair value measurement | Level 3 | Trading assets
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3,865	4,979	5,673	6,759
Recurring fair value measurement | Level 3 | Trading assets | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3	4
Recurring fair value measurement | Level 3 | Trading assets | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	674	934
Recurring fair value measurement | Level 3 | Trading assets | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1	1
Recurring fair value measurement | Level 3 | Trading assets | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3	3
Recurring fair value measurement | Level 3 | Trading assets | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Recurring fair value measurement | Level 3 | Trading assets | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Recurring fair value measurement | Level 3 | Trading assets | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3,184	4,037
Recurring fair value measurement | Level 3 | 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	10,404	9,476	8,245	7,080
Changes in FV due to refinement in application of levelling methodology		2,900
Recurring fair value measurement | Level 3 | Designated and otherwise mandatorily measured at fair value through profit or loss | 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
Recurring fair value measurement | Level 3 | Designated and otherwise mandatorily measured at fair value through profit or loss | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	98	28
Recurring fair value measurement | Level 3 | Designated and otherwise mandatorily measured at fair value through profit or loss | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	39
Recurring fair value measurement | Level 3 | Designated and otherwise mandatorily measured at fair value through profit or loss | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Recurring fair value measurement | Level 3 | Designated and otherwise mandatorily measured at fair value through profit or loss | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Recurring fair value measurement | Level 3 | Designated and otherwise mandatorily measured at fair value through profit or loss | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Recurring fair value measurement | Level 3 | Designated and otherwise mandatorily measured at fair value through profit or loss | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	550	578
Recurring fair value measurement | Level 3 | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3,855	2,136	$ 1,930	$ 2,423
Recurring fair value measurement | Level 3 | Derivatives | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Recurring fair value measurement | Level 3 | Derivatives | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Recurring fair value measurement | Level 3 | Derivatives | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Recurring fair value measurement | Level 3 | Derivatives | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Recurring fair value measurement | Level 3 | Derivatives | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	75	66
Recurring fair value measurement | Level 3 | Derivatives | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3,771	2,070
Recurring fair value measurement | Level 3 | Derivatives | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 9	$ 0