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, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Financial investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 350,002	$ 402,054
Recurring fair value measurement | Trading liabilities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	89,637	75,266
Recurring fair value measurement | Designated at fair value
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	151,686	157,439
Recurring fair value measurement | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	200,156	303,001
Recurring fair value measurement | Financial investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	350,002	402,054
Recurring fair value measurement | Trading assets
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	260,250	231,990
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	49,120	45,553
Recurring fair value measurement | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	209,516	307,726
Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	21,417	20,300
Financial liabilities	11,515	9,656
Recurring fair value measurement | Level 3 | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	206	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	7,237	5,330
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	3,421	4,187
Recurring fair value measurement | Level 3 | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	651	135
Recurring fair value measurement | Level 3 | Trading liabilities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	857	162	$ 105	$ 53
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	206	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	0	29
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	651	129
Recurring fair value measurement | Level 3 | Designated at fair value
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	7,237	5,306	5,008	5,016
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	7,237	5,301
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	0	5
Recurring fair value measurement | Level 3 | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	3,421	4,188	3,725	2,302
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	3,421	4,187
Recurring fair value measurement | Level 3 | Derivatives | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	1
Recurring fair value measurement | Level 3 | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	13,129	11,905
Recurring fair value measurement | Level 3 | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,167	1,834
Recurring fair value measurement | Level 3 | 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 | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Recurring fair value measurement | Level 3 | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	52	68
Recurring fair value measurement | Level 3 | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,625	2,602
Recurring fair value measurement | Level 3 | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	4,444	3,891
Recurring fair value measurement | Level 3 | Financial investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3,243	3,654	3,339	3,218
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	590	930
Recurring fair value measurement | Level 3 | Financial investments | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	900	1,286
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	1,753	1,438
Recurring fair value measurement | Level 3 | Trading assets
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,509	2,499	3,865	4,979
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	55	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	250	523
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	0	0
Recurring fair value measurement | Level 3 | Trading assets | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
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	2,204	1,972
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	12,988	11,477	10,404	9,476
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	12,484	10,971
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	17	25
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	0
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	487	481
Recurring fair value measurement | Level 3 | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,677	2,670	$ 3,855	$ 2,136
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	52	68
Recurring fair value measurement | Level 3 | Derivatives | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,625	2,602
Recurring fair value measurement | Level 3 | Derivatives | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 0	$ 0