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, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Financial investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 346,887	$ 344,767
Recurring fair value measurement | Trading liabilities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	94,149	84,431
Recurring fair value measurement | Designated at fair value
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	165,104	148,505
Recurring fair value measurement | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	229,903	205,835
Recurring fair value measurement | Financial investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	346,887	344,767
Recurring fair value measurement | Trading assets
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	271,424	238,130
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	41,043	41,111
Recurring fair value measurement | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	233,621	207,825
Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	16,553	16,674
Financial liabilities	7,478	7,142
Recurring fair value measurement | Level 3 | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	7	12
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	5,438	5,374
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	2,014	1,755
Recurring fair value measurement | Level 3 | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	19	1
Recurring fair value measurement | Level 3 | Trading liabilities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	49	58	$ 75	$ 93
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	7	12
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	42	46
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	0	0
Recurring fair value measurement | Level 3 | Designated at fair value
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	5,396	5,328	4,355	4,107
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	5,396	5,328
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	0
Recurring fair value measurement | Level 3 | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	2,033	1,756	1,870	1,949
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	2,014	1,755
Recurring fair value measurement | Level 3 | Derivatives | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	19	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	6,746	5,553
Recurring fair value measurement | Level 3 | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,245	2,202
Recurring fair value measurement | Level 3 | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	44	49
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	55	65
Recurring fair value measurement | Level 3 | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,875	2,358
Recurring fair value measurement | Level 3 | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	5,585	6,444
Recurring fair value measurement | Level 3 | Financial investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,363	2,000	2,228	1,767
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	611	427
Recurring fair value measurement | Level 3 | Financial investments | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,116	1,030
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	636	543
Recurring fair value measurement | Level 3 | Trading assets
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	5,673	6,759	6,547	5,080
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	10	20
Recurring fair value measurement | Level 3 | Trading assets | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,099	1,140
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	0
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	4,560	5,596
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	6,587	5,492	5,006	3,958
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	6,125	5,106
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	30	32
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	43	49
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	389	305
Recurring fair value measurement | Level 3 | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,930	2,423	$ 2,470	$ 2,444
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	55	65
Recurring fair value measurement | Level 3 | Derivatives | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,875	2,358
Recurring fair value measurement | Level 3 | Derivatives | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 0	$ 0